I was speaking with a friend today regarding Save the Dates - and he thinks it should be just as formal as the invites. ALL my research shows that save the dates are very informal (even if having a formal wedding). He suggested that I have EVERYTHING that is going to be on the invite on the save the date.
I'm not really sure of this, but wanted to ask.
From the Knot:
Your wedding save-the-date wording doesn't have to be anything exceptionally formal. Something like "Save the Date: [bride's and groom's names] are tying the knot on [your wedding date] in [the city where you'll marry]" is sufficient. This way, guests know what to expect, and they'll get in touch with you if they have questions. Some people include a short poem or a favorite lovey-dovey quote if there's room. This is the time to be less formal, so feel free to get creative.