It has to be a 365 day period. Depending on what method you choose, it can be a rolling or an entire year. So April - April is fine.
You can file either as bonafide resident, in which case you can return to the US for more than 35 days within that time frame, or on a physical presence test, in which case you can not have returned for more than 35 days.
If you meet the physical presence test, you can file your 2005 US tax return on or after April 30th (the end of the first rolling year).
If you file as a bonafide resident, you can not file until January 2007, to allow one full calendar year to have elapsed. If you use this method, you have to apply for a special extension (not the usual kind). Your accountant should know about this form, if s/he doesn't, get an accountant with more foreign experience.
Enjoy living here! I sure do.