This article is about the event. You may be looking for the episode.
Princess Day, similar to Grand Meeting of Ooo Royalty, is a meeting among all the rulers of the kingdoms, except that almost all the participants are princesses. According to Breakfast Princess, only real princesses have been invited to the event, concluding that those who were not shown are unregistered princesses.
The topics discussed were about the problems that each princess had encountered in her kingdom and trade deficits among the kingdoms.
It first appeared in the titular episode, "Princess Day," where Lumpy Space Princess was kicked out due to being a "bum" and insulting Breakfast Princess, which she calls back to LSP a "Pamplemoose" when Breakfast Princess calls the Maple Guards to get her out. Later, after one of the Maple Guards announced the problems that LSP and Marceline had created, Breakfast Princess leaves Strudel Princess in charge, stating that she would handle the situation.
Though it is said only real princesses can attend, Marceline is seen at the event despite being a queen. Likewise, Cinnamon Bun, who is male and only Flame Princess' consort, was also allowed to attend the event.
Although the event was held at Breakfast Kingdom, Toast Princess was not seen.