Sherry: Niles, before I forget. About your banquet - are we talking fancy-schmancy?
Niles: Well, it's not exactly the way it's worded on the invitation, but yes...
- Frasier, S4E9

The dress code for the wedding is Black Tie, as specified on the invitation. This is dinner jackets/tuxedos for the gentlemen, and evening dress for the ladies.

For those that are still confused, Emma recommends this article.

The ceremony is religious. There are no obligations but women may feel more comfortable covering shoulders, for the ceremony. Religious married Jewish women often cover their hair but this is not necessary for other married women.

Kippot (skullcaps) will be provided for all men to be used for the ceremony and any other prayers.