From the DC Board of Elections website:
Can I register and vote on Election Day?
Yes. If otherwise qualified, you may register at your precinct’s polling place on Election Day and cast a special ballot that same day. You may also register and vote a special ballot at any early voting center (the Board’s Office or satellite locations).
Whether you register at your precinct’s polling place on Election Day, or at an early voting center, you will need to complete a voter registration application, swear or affirm that you are a qualified elector, and provide valid proof of residence.
Valid proof of residence is either a copy of a current and valid government photo identification, or a copy of a current utility bill, bank statement, government check, paycheck, or other government document that shows the name and address of the voter.
NOTE: If you are previously registered, you may not change your party affiliation either on Election Day or at an early voting center.