WHO: Everyone who has ever been friends with Susan. WHAT: A party. WHEN: Today's Charms Club meeting.
As Zacharias promised, he arrived before the start of Charms Club to set up Susan's surprise party (though since it's Zacharias, Susan probably had an idea that this was going to happen). I assume that he got help from some of her friends, but there was cake, and a banner, and when Susan walked in the room, she MIIIIGHT have been deluged with glitter.
At Zacharias's grumpy, somewhat reluctant insistence, they were all badgered into singing her happy birthday. Other than that, people are free to... have fun things. Zacharias doesn't really know how to have fun.
After the party, he gave Susan her gift: two tickets to a Wand Direction concert in Leeds over Christmas holiday. He told her that she was under no obligation to take him with the second ticket. No, please don't take him with the second ticket. They are decent tickets, but not like, amazing front row backstage pass tickets. There was also a bag of Every Flavour Beans.