* Holiday is a common noun, meaning it describes a general category (a day off from work or school).
* Proper nouns are specific names of people, places, or things.
To make "holiday" into a proper noun, you need to specify which holiday you're talking about:
* Christmas
* Thanksgiving
* New Year's Day
* Fourth of July
* Hanukkah
* Eid al-Adha
...and so on. Each of these are specific holidays, and therefore proper nouns.