I'll echo the others, get a decent amount of sleep, do your rehearsal, and then: just do it. At the show, stay hydrated but skip the beer. And also skip flirting with the lovelies or anything else that might distract you from taking care of business. Just do your best to stay focused until you're off the stage. And try not to obsess too much over it in the process.
Some observations:
1) They've been together 5 years so they all know each other's quirks - except maybe yours. So they're 80% there on syncing with each other.
2) It's Top 40 - so you all pretty much know how the songs go. And that would be true even if you never played them.
3) You nailed the audition - so you and they both know you can handle the gig.
4) The audiences don't care (and usually can't even hear) 90% of the mistakes that send good musicians into a tailspin. And a good number of the audience will also have a few drinks in them and notice even less than usual. So that's an even bigger safety factor.
5) Festival audiences aren't there to do a critical appraisal of the music they're hearing. They're there to have a good time, bop, and dance. So keep the groove sweet and steady - and give them "good boogie" as the old saying goes. They won't (or usually won't) mind a rough start or mistakes in the song itself. Pay close attention to your endings as best you can however. If you have to focus on anything at this point, focus on nailing the endings as best you can. Like horseback riding or a bike, the place you're most likely to get hurt is on the dismount. A rough start is ok because you soon get your balance and it's smooth sailing after that. Once you're rolling - you're rolling, so that takes care of itself. A good clean dismount, however, puts a professional finish on the piece - and can make your group sound tighter than it is sometimes. So try to nail the endings. Note: if you flub it big time - and have a good frontman - he/she can often turn it onto a joke. "Just part of the act folks! Just part of the act!" I saw Tab Benoit do that by acting like he maybe had half a drink too many during the break - and the audience (of which half were fairly smashed at that point) LOVED IT!
6) Position yourself where you can constantly see the high-hat. That's usually the backbeat. When in doubt, always feel and work off the backbeat. It makes life a lot easier when you're the bassist. And it sounds better too
7) Stop worrying. To your earlier point - it is what it is. You've all been playing for a number of years, know the songs, and want to do your best. I'm sure it will all turn out just fine.
If it's any comfort, the first really serious gig I played was with an all original group that had 28 songs of their own. It was two guitars, drums, bass, lead vocal, and the rest of the group doing backup vocals. This group had already been together for two years when I replaced their original bass player after a major squabble. These guys (all accomplished and experienced musicians) had been writing material as a group - and rehearsing pretty heavily from day one. They got together at least three times a week for 4 to 6 hours at a stretch. (I got familiar with driving myself home absolutely exhausted between 1:00 and 3:00AM when I was with those guys.) So needless to say the arrangements were very tight. And to make it more fun for me, they didn't sound like anybody else's music I knew.
I only got about 30 hours of rehearsal in with them before the first gig. I brought chord charts on index cards for ten of the songs I was still shaky with and taped them to the top of my amp that first gig. The only time in my life I ever needed to do that.
Short story? I though I sucked royally that night. The band thought I did great. And the audience had a really good time. Good enough that the band got booked back by the club at the end of our second set.
Moral? Do your best and it will work out fine.
And remember...energy, a solid beat, and the desire to show your audience a good time, will trump flawless technique every time.
Luck!
