Wouldn't that be considered general culture though, considering most people in that country still celebrate those holidays?
You can not believe in that particular religion, but still it probably wouldn't hurt anyone knowing the reasons & beliefs behind those particular days.
They will not be forced to attend church or believe anything, but they will learn what a church is or the related traditions. Probably some other such as orthodox, Jewish and Islam basics.
I'm pretty sure Danish religious and cultural education is quite even handed compared to others such as say Polish.
Have a look at https://github.com/AsahiLinux/docs/wiki/Feature-Support