When Peter meets Donna on Valentine's Day, he offers her an egg but she places it back in the basket for safekeeping. Then when Irontail turns all the eggs green, Donna sadly replaces her own now-green egg apparently without having retaken it. Also it is not clear whether the egg given to Esmeralda on Halloween was also affected.
The Moon is shown full on Halloween (Oct. 31) and again on Christmas (Dec. 25). However, since the lunar cycles are 29.5 days, a full Moon on Oct. 31 would have been followed two cycles later by one on Dec. 29, not Dec. 25.
During the reprise of "When You Can't Get It All Together, Improvise", we hear Antoine singing, but see Peter Cottontail mouthing the words.
Antoine asks "in the rules of April Valley, does it say that the eggs must be given on Easter?" to which Peter replies no. Peter then eventually gives away eggs on St. Patrick's Day. But Colonel Bunny said earlier that whoever delivers the most eggs on Easter becomes the new ruler of April Valley and Chief Easter Bunny. Thus, Peter shouldn't have won at the end.
Irontail declares a law requiring all eggs to be colored dark brown or gray, but when he casts his evil spell on Valentine's Day he turns Peter's eggs green instead -- violating his own law and incidentally setting up Peter's chance on St . Patrick's Day. It's almost as if Irontail were more devil's advocate than truly evil.
Peter is traveling through the holidays in the yestermorrow-bile, but it's never explained how Iron Tail is able to follow him without a time machine of his own.