Party of One

After a successful party celebrating the first birthday of Gummy, her pet alligator, Pinkie Pie invites her friends for an "After Birthday" party the next day. However, they claim to have other errands to run, declining the invitations. She finds out these so-called "errands" are just an excuse to avoid the party. Have they grown tired of the pink pony's frequent celebrations, or is there something bigger involved?