These cards especially come in handy if you are planning a wedding in June, September or around the holidays, since they are traditionally busy times of the year. A save the date card should be sent out as early as possible. This allows your guests to avoid making plans for that day. Many invitations come with a specific save the date card that coordinates when the celebration will take place. Choosing coordinated save the date cards and invitations means making a decision early on about which design you are going to use. It also adds a nice level of customizations to your invitations.
This will vary a bit depending on whether parents are issuing the invitation or whether the couple are doing so but in general this style is acceptable: