In greetings and good wishes, the ending of the adjective depends on the gender (or
number) of the noun that follows, e.g.
Good evening.
Gute Nacht.
Good night.
Schönes Wochenende.
Have a nice week-end.
Frohe Weihnachten.
Merry Christmas.
Note: the words "viel" or "viele" in good wishes: "viele" is countable, "viel" is not, e.g.
Viel Glück! (Good luck!)
Viel Spaß! (Have fun!)
Viel Erfolg! (I wish you success!)
Viele schöne Tage in Berlin! (Have a lot of nice days in Berlin!)
photo of snowman: © Tim Bartel: https://www.flickr.com/photos/avatar-1/68970048
other photo: (CCO Creative Commons): https://pixabay.com/en/good-morning-board-chalk-coffee-1186350/