The word is distinctively Christian in origin.
Interestingly enough, while the word has been adopted by secular culture, its origins are distinctively Christian.
According to the Merriam-Webster dictionary, the word is derived from the “Old English hāligdæg, from hālig holy + dæg day.” Literally speaking a “holiday” is a “holy day.”
Historically this always referred to specific holy days in the Christian liturgical calendar, Christmas being one of the highest holy days of the year.
Over time the original meaning was lessened as holy days became associated with vacations and time off of work. Typically Christians would not work on these days, so rest and relaxation were a natural part of every holy day.
This is one of the reasons why “going on a holiday” in Britain is the same phrase as “going on a vacation.” At one point in time this was always associated with a specific Christian feast day, celebrating the life of a saint or even in the life of Christ. Currently the association no longer exists and “holidays” in Britain are the same as “vacations” in the United States.
The next time you wish someone a “Happy Holiday,” remember the Christian origins of the word and try to sanctify such days, offering up to God your rest and relaxation.