The calendars through which the time machine falls into the various holidays are all correct for the year the film was released, 1971 ... including New Year's Day and the holidays following, in which (if the holidays were visited in sequence) the calendars should have rolled over to the next year.
The name "Peter Cottontail" comes from a series of books by Thornton W. Burgess (1874-1965).
Antoine is supposed to be taking Peter Cottontail back in time to the previous Easter, but the yestermorrow-bile's malfunction apparently takes them forward in time instead... At Mother's Day, the boy tells him, "You forgot us on Easter." Donna tells him at Valentine's Day that she read about his mistake in the paper. Peter leaves Antoine at Christmas, and he has had time by the finale to turn into a butterfly. The finale therefore takes place at Easter a full year later. Iron Tail has presumably been so busy over the past year following Peter through time, that he never had a chance to implement any of those nasty things he said he was going to do to April Valley.
On the Yester-morrow-bile, the buttons for Halloween and Thanksgiving are not singnified with an H and a T, but rather an ST and SG. Antoine is French, and in France Halloween is called All Saints Day, which is signified with the ST. Thanksgiving, however, is an SG to make fun of how he pronounces the holiday in his French accent.
The music that plays in the background of the Halloween scene was originally used in an earlier Rankin/Bass production, Mad Monster Party? (1967).