Save the dates (also called STDs or Save the Date cards) are cards sent to potential weddingguests ahead of formal invitations. Their purpose is literally to ask guests to "save the wedding date" and not book another event on that date.
Save the dates are usually sent out six months to a year before the wedding. They are especially important for a destination wedding, a wedding with a large number of out-of-town guests, or a wedding around a holiday. Save the Dates provide guests sufficient time to make travel arrangements, and may even include information about the nearest airports and hotel blocks.
It is proper etiquette that anyone receiving a Save The Date will also receive a formal invitation.