northern hemisphere 

Hello there,
look at this sentence, there is a (useless) triple enhancement:

"Nowrūz is the traditional Iranian new year holiday celebrated by Iranian peoples, having its roots in Ancient Iran."

It's great to make reference to the origin of a festival, but this seemed a little overprotective (??!).

I would keep the reference to the origin (having its roots in Anciant Iran) and change the sentence in the following way:

Nowrūz [Persian: ...] is a traditional new year holiday having its roots in Ancient Iran and being celebrated by various people in the northern hemisphere, partly as the day marking the beginning of spring.

Yours,