AS THEY SAY down at the District's election headquarters, all's well that ends eventually. With the certification of the Sept. 12 results as well as concessions from Mayor Walter Washington and Sterling Tucker, the entire metropolitan area can now start thinking about the November elections, which we already know will involve replacements for several big-cheese officeholders. Between retirements and primary defeats, it is a certainty that Virginia will have a new U.S. senator, Maryland a new governor, the District a new mayor and city council chairman and Montgomery County a new chief executive.

Still other contests involving incumbents and challengers will determine who holds the top job in Prince George's County and who represents the area in Congress. And Virginians who enjoy a good race will decide the odds on parimutuel betting in the state, one of a number of referendum questions on the ballots.

But if any of those decisions interest you, there is another decision you may need to make: Anyone who isn't registered to vote and wants to reserve that option had better do so right away. In Virginia the last day to sign up is Saturday, Oct. 7; in the District, it is Sunday, Oct. 8; and in Maryland, it is Monday, Oct. 9. And, without dredging up bad memories from the District's vote-count confusion, we hasten to note that even some people who are registered - but who had to vote challenged ballots because they moved to different precincts - may wish to set their records straight and thereby help the cause of a better election count next time out. Who knows? - if enough voters get squared away now, there might even be certified winners in the District by Thanksgiving Day.