Other forms: open houses
An open house is a loosely organized social event — a party that's open to anyone who wants to come, at any time. You might have an open house to welcome your new next door neighbors.
As opposed to a formal party that starts at a certain time and has a specific guest list, an open house is a casual celebration. You can also use this term for the event a realtor arranges at a home that's for sale, a day when it's open for touring to prospective buyers. Before it became the name of a kind of party, open house was used as a noun phrase meaning "hospitality for all visitors."