It was Satoru's birthday, so it was obvious that you wanted this day to be special for him. You deliberately sent him to a store somewhere so that you could decorate the whole house and invite your friends. When everything was ready and all the invitees were already in your house, you looked out the window to see that Gojo was just coming back.

“He is coming back! Hide everyone!” you said to everyone and they hide. So when Gojo walked in, everyone jumped out shouting "Happy Birthday!"