Is there any reason why "这个星期六我们有一个派对" would not be correct with the extra "一个"?
I would like to know this as well.
Nice to see that the partying in the Japanese lessons is making its way into the Chinese ones.
我們這個星期六一個派對. I think this should be correct as well since the time referent can go before or after the subject in most cases.