Holidays are complicated. If you are involved in a faith tradition, you are told a holiday is available for you to celebrate, and in the case of some holidays, told ways it might be or must be celebrated.
It may not be what some adherents have in mind, but my current conclusion is that holidays are developed to set a day or days apart, in the context of some significant story or principle. With that, followers and nonfollowers alike are free to make of those holidays what they will, as long as they do so respectfully.
Shabbat on the Jewish calendar is one example. It comes around one day a week, a day representing the story of God’s completing creation. It is very holy on the calendar, second to none. Rules and traditions, including attendance at services, have developed for the day. There is a wide range of adherence to the rules among the faithful.
Which raises two questions. Are those who don’t observe the letter of the rules, sometimes acting widely far from the mark, any less in the spirit of the day than others? And if you are not Jewish, but deeply appreciate setting aside a weekly day different than others, why not?
The same goes for other holidays. If you want to spend ten days considering what you’ve done and how you might do better, the Jewish Days of Awe are for you. If you are enlivened by the idea that peace might arrive one day to a troubled world, in the spirit of an unexpected and unusual baby (you don’t have to buy the theology, but you might enjoy the colorful and heartfelt celebrations), why not?