Question: The new Hijri year is drawing near, the start being the month of Muharram. I am confused though – wasn’t the blessed Prophetic migration (Hijrah) in Rabi al-Awwal? If so why is Muharram the start of the Hijri year?
Answer: Yes you are correct, this being the case was answered by Hafiz Ibn Hajr (d.852) who explained: The Islamic calendar could have started from 4 possible places:
1. the date of birth of the Prophet (Allah bless him and give him peace),
2. the date of his being sent as a Prophet,
3. his migration or
4. his passing away.
As for the date of his  blessed birth and  sending forth as a Prophet, then there are differing views regarding the exact year when these took place, thus these were left as a starting point.
As for his  passing away, then it is a reminder of a time of sadness and was thus avoided.
This leaves  the Hijrah (migration) as the remaining option and this was moved from Rabi al-Awwal (its actual date) to Muharram. Why?
Because the pledge of allegiance to the Prophet (Allah bless him and give him peace) was taken in Dhul Hijjah and after this the firm resolve to migrate to Medina was taken. Therefore the first full month after the pledge and the resolve to migrate was in the month of Muharram. It was thus thought it most appropriate to start the Hijri calendar with this month.
And Allah Taala knows best.