You become a citizen as soon as you take the Oath of Allegiance to the U.S. In some places, you can choose to take the Oath the same day as your Naturalization interview. If that option is not available or if you prefer a ceremony at a later date, the United States Citizenship and Immigration Services (USCIS) will notify you of the ceremony date with a Form N-445, Notice of Naturalization Oath Ceremony.
If you cannot go to the Oath Ceremony, you should return the Form N-445, Notice of Naturalization Oath Ceremony, which the USCIS sent to you. You must also include a letter saying why you cannot go to the ceremony. Make a copy of the notice and your letter before you send it to the USCIS. Your local office will reschedule you and send you a new Form N-445, Notice of Naturalization Oath Ceremony, to tell you when your ceremony will be.