There are 64 hours between 5pm Friday and 9am Monday - and you want to make the most of every minute, not fritter them away on social events you really don't want to attend. Before you accept any invite, ask yourself.....
What's my gut reaction? If you tense up when the invitation arrives, that's your body's way of saying, I don't wanna go.
Would saying no be a big deal? Has turning down an invite ever caused friction with this person? It's doubtful passing up an everyday party would be a problem. (But blowing off your parents' 40th-anniversary bash? Not OK!)
How do I feel once I think it over? Maybe you're nervous about going solo. (Ask if you can bring a pal.) But if you're still not feeling up to it, RSVP a polite "No, thanks."
How do I want to spend my time? It's your choice! Say no to the big bash now and ask the hostess to dinner later, so you can enjoy the time together.