knees-up 
noun [in singular] British informal
A lively party or gathering: we had a bit of a knees-up last night...
- I love entertaining, and would rather have a dinner party than a knees-up any day!
- Whether you are throwing a grand soirée or a small knees-up in your front room, there is nothing worse than being at a party lit like a police station.
- One of the many events held last week to celebrate the Queen's Jubilee was a knees-up organised by Bedford Community Retirement Group.