the best time of day to practice tai chi

The most important thing is to practice regularly doing a little every day. Morning and evening have some advantages over other times, and other things being equal I'd choose mornings (or both).

Practice in the Morning

1. It sets you up for the day. You feel relaxed and energized. Practising early in the morning means getting into the healthy habit of 'early to bed, early to rise....'
   
   
2. You are reserving a time to take care of yourself. By making taichi part of your morning routine you reserve some time to look after your health before the pressures of the day crowd in on you.
   
   
3. The energy first thing in the morning seems particularly conducive to this form of exercise.
   
   
4. Avoids the heat of the day. In summer it is easier to practice before the weather gets too hot!
   
   
5. You should not exercise on a full stomach - well first thing in the morning you don't have to worry about that. You may even find that you feel less hungry and eat more healthily than you would normally

In the end you have to find what suits you best. So choose a regular time for exercise and try to stick to it and your body and mind will be ready for the daily session.

Practice in the Evening

1. The muscles may be freer in the evening and you may well find that some of the postures are easier to perform at the end of the day after the joints have loosened up
   
   
2. Evening is a quiet time and good for meditation. When the mind is calm your practise will be more rewarding.
   
   
3. Gentle evening exercise can help you have a relaxing night's sleep. This depends on the individual - some people find it too invigorating
   
   

As an instructor my biggest challenge is helping people to stay motivated. One final suggestion: Compare how you feel on days when you have practised and on those when you have not and hold on to the feeling. Remember the 'a-ha!' feeling.
Most of all enjoy your Tai Chi!