The best time to do Tai Chi is whenever you can. Traditionally Tai Chi practice has been recommended for the morning for several reasons.
1: The air is cleaner. This makes sense if you live in a polluted city, but it’s really very location dependent. In my rural community the worst air quality is on winter mornings after a windless night when many people rely on their woodstoves for heat – so go figure.
2: Morning Tai Chi practice sets the tone of your day with a relaxed body and a focused mind. That makes good sense; except that if you are a mother with children screaming at you then that relaxation isn’t going to happen. And it’s not always beneficial to just get up earlier, before the children. Sometimes you need sleep more than anything else, including Tai Chi practice.
3: Morning Tai Chi practice keeps you from letting the distractions of the day prevent you from making the time for your practice. Like all this advice, there is truth there. But that doesn’t mean your personal discipline isn’t better served by practicing at a different time of the day.
Tai Chi practice develops and grows with a regular daily practice. When you practice matters far less than that you practice.