You would be understood.
Technically that would be 'S e (remember that? As in, 'S e iasgair a th' annam). The course hasn't actually covered it yet, but 's e has a past and future tense as well. Don't panic, they are really really easy to learn.
So technically this phrase should be 'S e cèilidh mhòr a bhios ann.
The "bhios" (pronounced vee-us) is the bit which makes this future tense.
Conversely, if you wanted to make it past tense you would simply swap bhios out with bha (pronounced "vah").