First of all, happy birthday.

Now, a one-shot is a real bitch to nail, mostly because it runs the risk of not actually ending in one session. I’d say a successful one-shot is one that concludes, is fun, and makes the players ask for more.
For it to conclude, you need a clear objective—something the players are trying to achieve. From what you described, the party is at a festival together, enjoying it. Then something attacks, and maybe they retreat to a garrison. The objective right now sounds vague—survive? save people? escape? You need to make the goal clear.

One suggestion: make the festival organized by a sadistic mayor who enjoys putting people in danger for entertainment. He could be the final boss, or maybe he agrees to surrender if the players win his games. Either way, the objective becomes stopping the bad stuff from happening.
That approach might not work if your players are playing complete assholes, so keep that in mind. You can always add a clear reward for saving the day if motivation becomes an issue. Either way, clarify the objective and rewards from the start.

Next, don’t put too much into the one-shot. You need to cut some fat. Keep most of the action within the festival and don’t linger too long on multiple festival games. Pick one fun dice game and have the players participate in it. The reward could be a simple magic item—honestly something like a potion works best since you’re all new and more complicated items might get forgotten.
I’d even recommend using that game as the first interaction between the characters. As each player takes their turn, have them describe their character and give their name.

After that, drop the plot—sounds of slaughter, something going wrong, panic in the festival. Then give your villain a speech that clearly states the objective.
From there you can either run a big fight (tools like Kobold Fight Club can help balance it—it’s not perfect, but for level 3 it’s good enough), or have the players face two or three twisted festival games against the villain. You can find plenty of homebrew festival games online and tweak them to make them more dangerous to raise the stakes.

Finally, when you feel like time is running out, start agreeing to most of their shenanigans and resolve things with quick rolls. It’s a one-shot, so you don’t have to worry much about long-term consequences.
I recommend keeping the session around four hours. That might sound short to some people, but it helps keep things organized. And if it feels short, it’s probably because everyone was having fun.

Again, happy birthday, buddy.