iPhone calendar: removing Muslim holidays

You don't, because you can't. Subscribed calendars cannot be modified. If you don't want to celebrate a holiday other cultures do, just don't celebrate it. If you want a different calendar you can add holiday calendars or if you want, look for a Calendar App in the App Store, which you might have more opportunity to modify.

See this --> Set up multiple calendars on iPhone - Apple Support

MarmeeandLouisa wrote:

Obviously you don’t have to tell me not to celebrate it if I don’t want to even see it on my calendar. I’m not an idiot. Muslim holidays did not used to be on google calendars.

We really don't care what you celebrate or don't celebrate. The fact remains if you are using a SUBSCRIBED calendar, YOU CANNOT EDIT HOLIDAYS out of them. That's how they work, so that should end the discussion. If you don't want to use subscribed calendars go to the App Store and find a calendar app which lets you choose any holidays you want to enter.

Why are you asking about Google Calendars, this has nothing to do with Apple.

Contact Google for help with their services.