There are two.
The first is the Sun Valley Flyers contest on Feb 23-24 and the second is May 4-5 in Apache Junction. Beginning of May cannot really be considered "spring" in Phoenix unless you live there.
You can check the www.nsrca.us for contest calendar entries. Also the link to the district 7 site will get you the data.
Have you been to the SVF contest before?  I remember meeting someone over from England flying at the contest a few years ago.
John