does anyone know if there are training sessions of boston celtics the public can watch?

hello celtics fans and friends,
writing from poland, i am new to nba and celtics but it will happen that i go to see the celtics in march. it is a birthday present for my father who turned sixty. he wishes to get on a training session as well and i was wondering if this is possible. does anyone know and care about that? it´d be really great if you could help me or even inform me if such sessions are not offered. but if this sessions are offered, i would be of course keen to know how is is possible to get there, when, what it costs and what we need to do.

help greatly appreciated,
patricia