Buds will swell any time that the weather warms up after the plant's required chill period. In my observation, lilac buds are pretty hardy. However, I would by preference plant your lilac in the ground and mulched the ground around it well (though not against the trunk) if possible, as the general rule is that a plant needs to be about 2 zones hardier than the actual zone to survive in a pot over the winter and lilacs aren't zone 1/2 hardy. If you can't plant it for some reason, surround the potted plant with mulch that won't pack down, such as bark mulch or large trash bags of dry leaves to help protect it from temperature extremes and swings. If you don't either plant it or surround the pot with mulch, you may well lose it.

I was outside examining mine yesterday and tried to take a pic but the batteries dies - oh well. My lilacs are just starting to form flower buds now and they look like a teeny tiny bunch of dark purple grapes with normal leaves wrapped around them - so I would say that there is probably no way to tell before the little purple (if your lilac is purple - ha) flower buds come out. That I can see anyway.

yes.. they should have been there since about last august ... which leads one to wonder.. what you were doing out in this misery ... looking.. lol ... even if its budding early ... there is nothing you can do ... after last years misery ... most Ls of mine.. had lost the buds due to winter ... so i was looking forward to this years crop ... but for this winter.. which is worse than last .. crikey ken

Thank you gardengal48! These are all new to me so I wasn't sure when the bloom time was. My landscaper said I could prune them until mid-May so I guess that makes sense it would be a couple months until they bloom.