What's the deal with save the date cards?

If this is a typo, I love it. If not, I love it.

Another reason why STDs are useful is because weddings and other life-cycle events often come in waves. Sending out STD cards tells others in the same circle not to choose that same date because someone else has already chosen it and it’s set in stone (or may as well be, anyway).

could you use a better contraction please?

STD is the standard wedding-industry and wedding-speak contraction for “save the date.” It just is.

In the past it wasn’t unusual to invite some guests to “church only” and reserve invitations for the reception (then as now more expensive than the actual wedding) to closer friends & relations/VIPs. In really large homes with full staffs servants would be invited to the wedding proper (sitting either at the back of the church or in a balcony), then have to return home to serve the reception in conjunction with the caterers.