Is there a way to only show NYSE holidays, or hide non important holidays in icalendar?

If you can find a subscribable calendar that meets that criteria, add it and then turn the built in Holiday calendar off (so that it doesn't display). Neither a subscribed calendar, nor the built in Holiday calendar, can be modified.

Or simply add the holidays in manually to one of your personal calendars.

You're welcome.

And yes, it appears to have worked. Thanks