Warum sagt man "Fröhliche Weihnachten", wenn "Weihnachten" im Neutrum ist? Soll es nicht "fröhliches" sein?
If I remember correctly, it's originally dative plural, ze den wiehen nachten or something like that, which would be zu den ....en Nächten in modern German. (The adjective is no longer in use in modern German so I don't know what shape it would have taken. weihen, perhaps?)
But it's often treated as a regular plural noun, yes, hence Fröhliche Weihnachten!
On the other hand, Weihnachten steht vor der Tür can take a singular noun, perhaps because it's considered as similar to das Weihnachtsfest.