Based on the rema (YD 148:12) it seems that there is no issue. I would venture to say that you can probably say a generic “happy holiday “ even around Xmas time
The standard given is aivah/hate with most of the restrictions mentioned in the gemarah removed bc what the goyim practice now is not the avodah zarah meant in the gemarah.
The only real restriction still in effect seems to be going into their houses on the day of their holiday and using the word shalom (since it’s Hashems name)
The kitzur adds more things like supplying them material to help build a church
As for the link above. I can’t open it with my filter and there is no siman 148 in the SA Harav (what is the summary?)
Btw. Even holiday parties are mentioned as being mutar (but a baal nefesh should attempt to not go)
