Arabs used solar calendar.
Shamsi calendar by attabukharui
The old names of the months.
Quran, Ramazan n soam kia hain? Part 2 by attabukhari
Ramazan always in extreme summer month. Not dependent on lunar cycle.
[ for intensive military training ]
juma ka din kia hai
urdu –36 minutes
The Jews used to use lunar calendar. They may have used Umar [RA] name for the implementation of Islamic Calendar, based on lunar cycle.There was no Muharram, it used to be called Safar 1. !