Corrected entry: When Snoopy sets up the table, he puts plates out, topped with napkins. The last one was removed, due to Woodstock being under it, and not replaced.
Corrected entry: Linus, Sally, and Charlie Brown are talking about Thanksgiving coming up. Sally complains "I haven't even finished my Halloween candy!" Funny thing, considering that Sally sat with Linus waiting for the Great Pumpkin in the pumpkin patch, therefore missed out on the candy.
Correction: There's no reason to think the pumpkin patch incident (from a movie made 7 years before this one) happened the same year as this thanksgiving. Even if we assume this, just because Sally missed trick or treating and the party doesn't mean she didn't have Halloween candy (I have Halloween candy each year myself without ever going trick or treating or to parties).
Correction: While we don't see it replaced, plenty of time passes before the guest arrive. In fact, when the guest and Charlie Brown are walking out to the table, Linus and Sally are already standing by the table waiting and could have easily put the napkin back (if it wasn't Snoopy).