Classic advice on when to meditate ranges from “first thing in the morning” to “at the end of the day”, but ultimately it fails most people because it is assumed that establishing a routine is supportive, and that routine equates relatively fixed times.
This is more pronounced for beginners, as they'll easily read that “you may need some time to build a habit. You can try these different times, but I recommend trying one time for a week straight to see how it works. In order to find the best time to meditate, it is going to take some consistency.” While a routine is important, it is not about fixed time! Understand that if time might be promoted as a “trigger”, then other phenomena might too… more constructive phenomena in the grand scheme of things, and if you're to build a habit, a routine, then it might as well be as constructive in the long run as possible.
“Meditating in the morning is a great way to start the day”, or so says the usual website. It is usually followed by a lie, like “personally, I find that sitting in the morning helps me have a day with more awareness”; it is a lie (thus a breach of precepts for most Buddhists) simply because the benefits usually last until one skips breakfast and rushes to work… or until the first event that unsettles the mind and takes the awareness captive. It doesn't last a day, not even close.
There's another aspect of morning meditation which is not-so-helpful. It is linked to a search to make meditation convenient, and it arises with thoughts like “If you set the intention to sit right when you wake up, you can get your meditation done before the day gets going, and you can then (basically) forget about it: it's done.” This is “spiritual materialism”: not only it supposes that accumulation of meditation practice is good and postponment is bad, but also it supposes that it's better if it's convenient, pleasant, easy (because you're rested, fresh), non-disruptive…
Although “meditation” is a term often used in relation to Buddhism, the Pāḷi word is more akin to “cultivation”. The Buddhist path is a path of reform, of changing from the normal, usual neurotic self bound by habits, prejudices, opinions, preferences to unconditioned freedom! If we take it seriously, then it ought to be seriously disruptive, not just some minor adjustment to make our lives slightly more bearable.
Suggesting to meditate at the end of the day, the classic “meditation can help us wind down, relax, and settle in at the end of our days” is, of course, plagued by similar spiritual materialism. It's again about meditation being instrumentalised to make our lives easier without too much effort, or change, or questioning. It confuses cultivation with relaxation; if you see meditation as the study of your own mind, then studying shouldn't be confused with relaxing.
Additionally, it is commonly hindered by sleepiness, or by worries and an over-excited mind; and it is more easily postponed e.g. due to going out (“I'm just too tired today, I'm falling”).
The association of meditation with relaxation also makes it harder to meditate at other times. Ultimately, with such an approach, you usually train to relax i.e. to be less present, less aware of disturbances, more than you train to relate differently to disturbances (without losing your inner peace, without falling for unexamined reactivity, impulsive and prejudiced responses…).
Amida Nyoraï (Edo period)
Why do we sit to meditate?
Why do we take some chunk of time to do so?
We do so to train, and training takes time. But the point of training is never to stay stuck in the training. Think of it like dancing and martial arts: you might repeat movements for hours, in the search for the perfect gesture, the perfect timing, the appropriateness to the context at hand, and yet the proof is when you'll do the movement just once in the midst of circumstances that called for it, outside of the training!
The goal is not to meditate for hours on hand. We learn to refrain from automatically following unexamined trains of thoughts; we learn to refrain from reactivity; we learn to discern how small thoughts snowball into larger problems; we learn how to put the emergency brakes on panic, fear; we learn not to let fear distort the information; we learn not to lose sight of the bigger picture as soon as me, myself and Iam personally involved or at risk. And the point of such a training is to halt anger whenever it arises, not just on the cushion; it is to diminish our biases and prejudices in dealing with others, not just on the cushion; it is to reduce our projections and over-interpretations, not just on the cushion…
The goal is to instantly meditate, whenever it's useful to be aware, present, mindful, engaged, whenever bringing positive energy to the table is likely to yield better results, more constructive outcomes, than negative energy… i.e. all the time: the goal is to perpetually and instantly be constructive, wholesome, willing to help others as well as oneself.
So we might set aside periods to train, sure, but we need to keep in mind that the goal is outside of these periods. Meditation is one tool among many in the arsenal to cultivate wisdom (typically, “being present” is not enough in and of itself to ensure wisdom: one also needs, at the very least, to understand causality, i.e. how a “present” act is likely to unfold in the future…).
Then, we train to transition: we train to transition from automatic, instinctive, self-centred responses to wiser responses encompassing many more perspectives; we train to transition from annoyed neurotic wreck to “who I want to be” (usually not so much someone overwhelmed by e.g. a nasty remark or a bad surprise, but someone who can engage with calm and clarity); we train to transition from someone under the dictature of unexamined, subconscious impulses to the wise. And the point of this training lies whenever such a transition is called for, and we prove able to see it through.
Now, frankly, transitioning from sleepiness to wakefulness, or vice versa, is far from the most useful transition. It might be argued that it's better than nothing, and it is —after all, if you do know how to ensure not to fall asleep at the wheel, it's certainly beneficial for you and for others,— but that's just the tip of the iceberg and if you think this tip is useful already, then why wouldn't you aim for a bigger slice of the benefits?
Then, the right time to train is a transition from one activity to the next, from one context in which some concerns might be relevant to another context in which the same concerns are out of place, biases, in where they hinder you from responding to the situation at hand (e.g. anger can be carried from one context to the next, and you end up taking your frustration on the “wrong” people most commonly your loved ones, because you take for granted their forgiveness).
So the best time to sit and meditate for a chunk of time, as a training, is between home and outside (on the way out, or the way back), between one activity and another (e.g. between work and hobby; e.g. after work and before an evening out), etc: transitions!
The morning is thus not “meditate half-asleep, then rush to get out of the house, or lose calm as soon as one gets on public transport / hits the road” but “get ready and meditate before leaving”… The evening is thus not “come home, check homework, shout, rush diner, pay bills under duress, watch TV to try and relax, then make oneself fall asleep by shutting down the observer-awareness-mind” but “come home, switch context thanks to meditation, suspend work-related thoughts and annoyances and worries, then engage with an open heart and patience with the loved ones, cook with attention, do the necessities without seeing them as a burden but as an investment, enjoy one's evening and recharge the batteries”.
As you get better at focusing the mind, at applying the emergency breaks on vicious circles of thoughts, at switching contexts, you might multiply the opportunities for short(er) meditation sessions… e.g. one minute between entering the car and turning on the engine, one minute between opening the exam paper and calmly engaging with it, one minute between finishing a task and checking your emails… thus, gradually, getting into perpetual, instantaneous meditation: neither all the time on the cushion (with a fixed posture), nor never meditating when it would make the most difference.
At this point, you become able to re-focus on the breath when anger arises, avoiding the yelling or the annoyance or the stress and letting it go with barely a flash in the eye. At this point, you become able to stay calm, neither attack nor flee, under “stressful” conditions. You become like a first-aider, to yourself and others: you engage without paralysis but also without rushing, you collect information before seeking help (so you get the appropriate help), you check what others understand when you communicate something (to minimise misunderstandings e.g. regarding your location), etc.
At this point, meditation appears clearly as a profound reform in your life, and worth changing your daily routine a lot more than merely fitting the practice in a relatively convenient time slot.