I wonder, why does the Islamic new year start in the month of Muharram. As Muslims start their calendar with the immigration of the Holy Prophet from Mecca to Medina, known as Hijra, the first day of the year should be in the month (Rabbi ul Awwal) the Prophet left for Medina.
Both decisions, ie when to start the Islamic calendar and what its first month should be, were consulted upon and decided by the Sahabah [may Allah (ﷻ) be pleased with them], who agreed that it was the best to start the calendar based on the Hijrah and to start the calendar with Muharram as the first month.
Why did they choose Muharram over Rabbi Al Awwal? - Mainly because of its perfect/suitable timing in terms of following Dhul-Hijjah, the month in which Hajj is undertaken and more relevantly to the topic of migration - the month in which the stages for the Hijrah were being set to begin with.
"..they chose to regard Muharram as the first month of the year rather than Rabee‘ al-Awwal because the plan to migrate started to take shape in Muharram. The oath of allegiance (bay‘ah) – that was the precursor to the Hijrah – had taken place during Dhu’l-Hijjah, and the first new moon after the oath of allegiance and the decision to migrate was that of Muharram. So it was appropriate to make it the first month of the year.." (Statement by Ibn Hajar - IslamQA)
"After a discussion, they may Allah be pleased with them all agreed unanimously to adopt that the Islamic Calendar should start from when the Prophet sallallaahu alayhi wa sallam (may Allah exalt his mention) emigrated from Makkah to Madeenah and that the first month of this new Calendar be that of Muharram. This is because Muharram is a sacred month and because it follows Thul-Hijjah which is the month in which the Muslims perform Hajj, or the pilgrimage, which is the last of the five pillars of Islam that was enjoined upon the Muslims.
Muharram also follows the month in which the Ansaar (the Madeenan supporters of the Prophet sallallaahu alayhi wa sallam (may Allah exalt his mention)) pledged allegiance to the Prophet sallallaahu alayhi wa sallam (may Allah exalt his mention) which was prior to his migration to Madeenah. This pledge of allegiance was one of the introductory acts of the emigration itself and therefore Muharram was deemed the most suitable month to start the Islamic Calendar by the companions of the Prophet" (Islamweb).