What does it mean when someone says
AN EVENT IS ON
A PARTY IS ON?
When an event, party, or function is on it means it is going to happen, or going to take place, as was planned earlier.
This is informal English.
Are we still on for lunch?
(This means that earlier you discussed having lunch together, and now the speaker is checking if you are still going to have lunch together.)
The party is on for Saturday, the 8th. You are planning to come, right?