weekend warrior

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who enthusiastically engages in an activity or hobby on the weekends outside of regular employment.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A part-time soldier, or reservist
• noun idiomatic A person who indulges in a sport or pastime on an infrequent basis, usually on weekends when work commitments are not present.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a reservist who fulfills the military obligation on weekends
• noun a homeowner who acts as a contractor and tries to do major improvement projects on weekends (often without understanding the scope of the work to be done)