Usually, invitations are sent out about six weeks before the wedding.  This gives guests plenty of time to make travel arrangements and clear their schedules.  Invitations are normally issued by whoever is hosting the wedding.  There are many different ways to word your invitations.  (Examples) 

A traditional formal wedding invitation includes:

-The names of those hosting the wedding (traditionally the bride’s parents)
Mr. and Mrs. Albert Jones

-The request
Request the honor of your presence
at the marriage of their daughter

 -The names of the couple to be wed (formal ettiquitte includes full name)
Elizabeth Ann Jones
to
Robert James Schultz

 -The date and time (traditionally spelled out)
Saturday, the seventh of March
two thousand and eight
at three o’clock in the afternoon

-The location
Blissful Manor
872 Arlington Court
Richmond, New York

-Time for the Reception
Reception immediately following

You may include RSVP on the invitation, or enclose a separate reply card and envelope with the invitation. Guests should RSVP at least two weeks before the wedding.

Other options you might want to add to your invitation can include telling your guests what to wear.  Black tie, white tie, semi-formal, cocktail attire, black tie optional, informal, etc.  If you do not wish to have children at your wedding you might include Adult reception.

Save the Date cards are not necessary, but are a good way to let your guests know of your upcoming wedding date.  This gives your guests time to prepare for travel arrangements, scheduling issues, or taking time off work.  Save the Date cards tend to be less formal, so get creative!  Some couples like to include a poem, joke, quote or picture on their cards.