What is the significance of the Islamic calendar starting from 621 AD?

Prophet Muhammad (peace be upon Him) and his followers migrate from Mecca to Medina between June 21st and July 2nd in 622 AD.

Follower who took journey From Mecca To Madina called Muhajireen And

People Who welcomed Prophet Muhammad (PBUH) in Madina called Ansar (helper)

Migrated Because so that they can peacefully spread the Word of "there is no God but Allah"