Secular societies don't have to be based on Christian culture.

 Here is a list of holidays from Indonesia, the largest Muslim country, with over 200 million people:-
http://www.officeholidays.com/countries/indonesia/

They have holidays for Muslim, Hindu, Buddhist and Christian festivals.

Here is a list of holidays from Bangladesh, the third largest Muslim country, with over 160 million people:-
http://www.officeholidays.com/countries/bangladesh/2017.php

They have holidays for Muslim, Hindu, Buddhist and Christian festivals.

Would you like me to show public holidays for western/Christian countries, that also have these same religions?