Anytime, but the monsoons :)
It gets closed in monsoons. otherwise, winters is a good time as you wont feel hot in the jungle during safari. and animal spotting is nice. in summers, tiger spotting gets easier at waterhole.
Don't go to jim Corbett .. totally wastage of money . because there are only 5-7% chances for tiger that they are roaming in jungle .if you really want a safari ride then go to Ranthambore (rajasthan) . * by the way the best season for jim Corbett is October or April (no cold no hot)