Not always. Usually there is an extra page in the same envelope when you receive invitation for Test date, stating that you may be asked to join an Oath ceremony which will be held on the same day at that location. Otherwise, you will be advised about the Oath Ceremony by the interviewing officer after the test. Oath letter will be mailed separately or in rare cases the officer may hand out Oath letter for a specific date subject to approving your application on the spot.