open house

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

• noun A social event in which hospitality is extended to all.
• noun An occasion when a school or institution is open for visiting and observation by the public.
• noun A period of time during which a house or apartment for sale or rent is held open for public viewing.
• noun A house or apartment open for such viewing.

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

• noun A house habitually kept open to callers.
• noun An event, often in one's home, at which there are few restrictions regarding who and how many may attend.
• noun An event during which a facility or institution is open to the public for inspection or tour.
• noun Australia A period of time, usually several hours in a day, during which a house which is for sale is open for inspection or tour by interested parties.
• noun US A casual school event where the school invites parents or guardians to come and tour their child's school, classrooms, and allow them to meet their child's teachers.

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

• noun an informal party of people with hospitality for all comers