1. Mine was roughly 4 weeks but that's because I had a pending I-751.
2. In theory, you can't "select" a date for the ceremony; I believe all the oath ceremonies are conducted at the Heritage Theater in Campbell.
3. It is possible to reschedule the oath ceremony. You can get oath ceremony dates here and when you go to the interview, you can request a date if the next available one doesn't work for you. However, this is subject to the interviewer's discretion and doesn't necessarily mean you will get the date you want because said date might be completely full. They have 4 different ceremonies in one day with each having slightly over 400 people being sworn in. With your interview date being 3/19, if all goes well then you'd probably have the ceremony on April 18th.
