The German word "Weihnachten" (Christmas) is an odd one. It's a neuter noun (das Weihnachten) even though it's based on a feminine one (die Nacht, night). The traditional Christmas greetings, "Frohe Weihnachten" or "Fröhliche Weihnachten," don't follow the rules for singular neuter nouns, though they'd make sense if it were a plural (think of the 12 days of Christmas). The plural of "Nacht," though, is "Nächte." Some German nouns add "-en" as a plural or for non-nominative singular cases, but that never happens with feminine nouns, and neuter nouns partially follow it only when they have certain endings (e.g., das Museum / die Museen). The dative plural for "Nacht" is "Nächten," but the umlaut is mandatory.
As a further complication, the combining form changes the "en" to "s" (Weihnachtsmarkt, Weihnachtsabend, and many others). The combining form of "Nacht" is just "Nacht-".
Does "Weihnachten" fall into some grammatical category that lets this all make sense, or is it a unique instance, built from dialects and archaic usage?
