Even though your choice of what to serve really doesn't require an explanation, it's not a bad idea to give your guests a heads up about alcohol, says Post. The best way is by word of mouth, she says; ask your family and your wedding party to pass along the information as they would any other details about the wedding and reception. You could also casually mention it to guests with a simple "By the way, there won't be any alcohol served at the reception."
If your guest list is large or you prefer to let people know in writing, add a discrete note on the reception page of your wedding website: "Please know that alcohol will not be served." The same wording could also unobtrusively appear on any insert you include with your reception invitation, such as directions to the venue or other logistics, but Post says this should be a last resort. Never include it on the wedding invitation itself.