You should send save-the-date cards to all of your intended guests when either of these circumstances fit your situation. Even friends and family members who you already know will attend should be sent a card.
Save the date cards can take the form of any message medium, and often do. They sometimes correspond in style to the wedding invitations, or they take the form of refrigerator magnets, brochures, postcards, booklets, or tri-fold letters. The medium is up to you, but the shortness of the message usually lends itself well to magnets or small cards.
If you are going to use cards and stamps, you might consider creating your own stamp. You can make these stamps online and use your engagement picture. It’s fun and it makes it very personalized.
To save yourself time with licking the envelopes, you might consider stickers. You can purchase round stickers with your last initial or a rose or heart. These can be found at the stationery store or where you are purchasing your invitations. Another option is to purchase glue sticks to seal the envelopes.
Another well received way is to print a mini calendar page with a 3-hole punch so your friends can attach it to their own calendars. Of course, you would have to circle or decorate the day that belongs to you.