["I'm having a party this weekend and I already got my grocery shopping done for it! I refuse to be last minute like last time.", "I have a friend who plans ahead like that.  I'm a last minute gal.  I think you will be less stressed by planning ahead.", 'I will be and be able to enjoy myself more in the days leading up to the party.']
Not to mention enjoy the actual party, too!  Are you celebrating anything in particular?
I'm having a party this weekend, I went out and got the food ahead of time.