My advice: Put holidays in your writing as often as fits your plot. I say this for several reasons. For one thing, people identify with holiday celebrations, and (speaking for myself) often compare the rituals described with those from childhood.
Perhaps a more important reason is that emotions run high during holidays—for good or ill! And those emotions are a great source of tension both within characters and among them.
Virtually any holiday can evoke virtually any emotion: sadness, mourning, joy, anger, frustration, fatigue, relief, etc.
A second piece of advice: Include something novel or unexpected. When writing about any given holiday, there is a tendency to draw on one’s own experience—not that there’s anything wrong with that! But holiday rituals tend to be just that: ritualistic. So if your writing includes the same holiday more than once, you will need new material.
A third piece of advice: Have this book on your shelf.
This book is an incredible resource, a combination of calendar, dictionary, and cross-referenced guide.
First, it goes by date, so if you need an out-of-the-way holiday to fit your timeline, you’ll find it here.
Within each date, entries are alphabetized by relevant country. For example, New Year’s goes from Albania to Yugoslavia. This is one good way to include a description of your character’s ethnic background.
Another great way to flesh out your character’s ethnicity is to look up the country in the index, where you can find all the dates when holidays are celebrated in that country—and what they are, of course.
The index is extremely well done. Besides by country, you can search by person or topic. And the topic can be a standard one, such as songs or food, or a less common theme, such as animals from birds to sheep.
If you are really into one particular holiday, there are a plethora of specialized references out there. But The Folklore of World Holidays has 50 pages on Christmas. It’s likely to meet most of your holiday reference needs. Ask Santa to drop one under the tree this year!