field day

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

• noun A day set aside for sports or athletic competition, as at a school.
• noun An outdoor meeting, social event, or festivity.
• noun A day for military exercises, maneuvers, or display.
• noun A time of great pleasure, activity, or opportunity.

from The Century Dictionary.

• noun A day when troops are drawn up for instruction in field exercises and evolutions.
• noun Hence Any day of unusual bustle, exertion, or display.
• noun A day when explorations, scientific investigations, etc., as of a society, are carried on in the field.

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

• noun military A parade day.
• noun A school day for athletic events; a sports day.
• noun A day of class taken away from school for a field trip.
• noun idiomatic A great time or a great deal to do.
• noun idiomatic A great time or a great deal to do, at somebody else's expense.
• noun Top-to-bottom all-hands cleaning.

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

• noun a day for outdoor athletic competition
• noun a day devoted to an outdoor social gathering
• noun a time of unusual pleasure and success
• noun (military) a day for military exercises and display