"On Christmas Day" means December 25th, but otherwise these are not fixed time periods that start and end on specific days. Most of the time you can figure this out from context. For example:
Here's how you can spend more time with your family and less time shopping this holiday season
Because Christmas day is often a work holiday for many people, "this holiday season" means "around December 25th this year", but when it starts and when it ends varies. For some in the United States, the season can start right after Thanksgiving, which this year was November 23rd, because that's when they start preparations for Christmas Day. The season usually ends on January 2nd, because that's when many people go back to work.
However, other people may consider "the holiday season" to start and end earlier or later, depending on religious faith and personal preference. Orthodox Christians celebrate Christmas starting on January 7th. Jewish people celebrate Hanukkah starting December 13th. And so on.
When someone in a book (written about people living in the Americas or Western Europe) says, "We always break for the holidays at Granny's around Christmas," you can assume this means some time period which includes December 25th. Unless there is more information from context, the actual start and end dates are probably not important.