Salam alaykum wa rahmatullah Sidi,
Did Rasoolullah صلى الله عليه وسلم arrive in Madina in Muharram or Rabi’ Al-Awwal?
Waalaykumussalam warahmatullah wabarakatuh
Rabīʿ al-Awwal per all the early sources including Bukhari in his Sahih (Manaqib al-Ansar, Hijra). They made the Hijri calendar begin in Muharram because that is when the intention of Hijra was made.
Hajj Gibril Haddad