I wrote this for a similar question earlier today:
My understanding is that Principal Appointments are made through April. Appointments must be accepted by Acceptance Day the first week of May (not sure the exact date this year).
In previous years, there have been about 100 Principals who have declined or have not responded by acceptance day. A second round of appointments will go out in May to fill the declined positions. This can go into June.
Last year my DS was notified via email from the recruiting officer April 29, a Saturday, that he was accepted. The BFE came Monday. He accepted via email and by mail just to be safe.
My DS was fully qualified on hold for about 6 weeks prior to his appointment. I know how you are feeling. Once the appointment comes, get ready for the ride! It comes fast.