Hello, Muhammad, and welcome to the Grammar Exchange!
You should use "on," because "take on" means "hire."
Although "Christmas period" sounds fine in headlines and announcements, I'd prefer "the Christmas period" within the context of a sentence. Compare:
- Sales assistants needed for Christmas period. Apply now!
- The store always takes on extra sales assistants for the Christmas period.
Note: Next time you ask a question, please try to start the thread by using a title that is more descriptive of your doubt, for example: Phrasals with "take"