Why did Omar ibn al-Khattab adopt the Hijra as the beginning of the calendar, and why does the Hijri year begin with Muharram while the Hijra took place in Rabi' al-Awwal, and what are the connotations of the names of the Hijri and Gregorian months?

Umar ibn al-Khattab established the Hijri calendar after consulting with the Companions, who unanimously agreed upon its selection. This was done to differ from the polytheists and to distinguish the Muslim nation, and because Islam emerged and its state was established with the Hijra (emigration). The month of Muharram was chosen as the beginning of the year because Muslims would gather in Mecca during the months of Hajj and disperse in Muharram. As for the names of the lunar months, the Arabs derived them from their environment and the times in which the naming occurred, such as naming Shawwal after the "tashweel" (drying up) of camel's milk. One can refer to linguistic dictionaries to trace the names of both the solar and lunar months.