There is "Season's greetings" or "Merry Christmas" for Christmas. But is there something for Halloween? "Happy Halloween" just does not sound right to me because of the contrast between "happy" and the "pseudo-horrifying" nature of Halloween. And neither does wishing a "horrifying Halloween" seem appropriate. (I'm neither a native speaker nor located in a country with English as common speech.)
Greetings – Season’s Greeting for Halloween
greetingshalloween
Related Solutions
How do ye / How do you do / Howdy?
From the etymology.com page on howdy:
howdy 1840, first recorded in Southern U.S. dialect, contraction of how do you do (1630s), phrase inquiring after someone's health; earlier how do ye (1560s).
Note that a search for "how do you do" in the Corpus of Historical American English (COHA) (1810s - 2000s) in particular brings results that are from the early 1810s (see number of occurrences per decade below). This supports that the usage of "how do you do" precedes the year in which Americans started greeting with "hi" according to etymonline.com (1862).
Similar searches in the COHA for "how do ye do" and "howdy" support that also their usage was prior to when "hi" started being used as a greeting.
Hal / Hail
Disclaimer: no evidence that these words were used in America. What follows is more like an interesting note on the history of greetings in English:
The book Speech acts in the history of English dedicates an entire chapter to greetings in English language history. Among the greetings that it covers, I would say that the closest one to hi! in spelling and usage is hail! The author describes hail as the Middle English daughter form of the Old English interjection hal. Hal! would literally mean health!.
You can browse the book here in Google Books.
"I hope you had a nice Christmas" or "I trust you had a nice Christmas" would both suffice. The latter might sound a bit overly-formal to some ears, but if it's a business contact you don't know outside of business that's not necessarily a bad thing.
Much is made in some quarters about whether it is better to refer to Christmas or the holidays generally, with differing opinions on both sides.
Personally as a non-Christian who does celebrate a different holiday near the same time of the year, I find nothing objectionable about people wishing me a happy Christmas - being happy at Christmas is definitely preferable to the alternative, after all.
Still, if you know that they celebrate Christmas or if they have mentioned Christmas previously as the reason they will be un-contactable for a while, then go with Christmas. If you know they celebrate another holiday (solstice and Chanukkah were both celebrated recently) then do mention the holiday they celebrated. If you're unsure then something like "I trust you enjoyed the break and are keen to start on the new year's projects" can avoid mentioning a particular holiday without sounding like you're avoiding it (there is a minority who consider anybody enjoying a holiday near Christmas that isn't Christmas to be a "war on Christmas" and get upset about avoiding being overly specific).
All that said though, "I hope you had a nice Christmas" can't go far wrong.
Also, if English is your second language and they know where you are from, a native greeting in your own language can be a nice touch, especially if context or similarity in etymology makes it easy to guess the meaning of.
Best Answer
Americans routinely say "Happy Halloween", as incongruous as it may be. Actually, if the day was really horrifying, we wouldn't be celebrating it: We'd be cowering in our basements. It's more a day to make light of supernatural evil than to be frightened by it.