Wedding rehearsal dinners are typically formal events, but the dress code can change depending on the hosts preference and the location selected. Additionally, your role in the rehearsal dinner may help decide what you wear. The wedding party may have to dress more formally in the event that professional pictures are going to be taken, while other guests can usually dress a bit less formally.
No matter where the event is held, a business casual outfit is typically appropriate. At a minimum this includes a long sleeve dress shirt, dress pants, dress shoes and matching accessories (socks, belt). If the event is more formal or you are part of the wedding party, a sport coat should be included.
If in the wedding party, check with the bride/groom for any special dress code
Dress appropriately for the location of the dinner
Remember there will probably be lots of pictures taken, so look your best