The phrase "to have a party" means to organize and host a gathering where people come together to enjoy themselves, celebrate, and have fun. Full definition
I am probably just going to do what he wants and then go have the party with everyone else on another day. (babycenter.com)
They don't ever have a party at school where they are permitted to consume their entire daily calorie allowance. (thelunchtray.com)
Say what you will, be as cynical as you like, it seems we now have a party in British politics who promise to lower the tax burden on the British people. (conservativehome.blogs.com)