I am not sure when to omit TO, for example,
You can watch TV whenever you want to or
You can watch TV whenever you want
which one is right?
Regarldess of whether or not you want, you should attend family occasons or
Regardless of wheter or not you want it, you should attend family occasions
Regardles of wheter or not you want to, you should attend family occasions
which one is right? or why?
Both are used. Note, words in brackets (...) are optional.
 You can watch TV whenever you want to (watch TV).
 You can watch TV whenever you want (to watch TV).
Choice stems from a prescriptivist rule that tells us never to end a sentence with a preposition. Mind you, "to" of "want to" isn't a preposition. It's a infintive marker, but it's homophonous with (sounds the same as) the preposition "to".
 Regardless of whether you want to (attend) or not (want to attend), you should (attend).