Regardless of how formally you are constructing your invitations, adding the word "cocktail" before "reception" is the most straightforward way to let your guests know that dinner will not be served. Traditional wording might read something like: "A cocktail reception will follow the ceremony" or, more casually, "and afterward for cocktails and hors d'oeuvres."
The start time for your reception will also help clue in guests. "Set the event during cocktail hours -- that can be 4 to 6 p.m. or 5 to 7 p.m. -- and anticipate that it's going to be a shorter reception than usual," says Mac Adam. Whatever time you choose, wrap things up, including the cake cutting, no later than 8 p.m. so people will still have time for dinner.