The Prophet Muhammad’s mission lasted 23 years. It began when he was 40 in his birth-city of Makka, and ended 23 years later in a city called Yathrib, which was renamed Madinah-tun-Nabi, or simply, City of the Prophet. The first 10 of those years in Makka were tough. This was a major trading city bustling with commercial power players, that is, authoritative Arab families, who blocked, banished and persecuted the few loyal followers Muhammad had gained.
The city of Yathrib, about 200 miles away, was different. Locals had heard news of this “Messenger” person in Makka and felt their city needed a leader like him. Muhammad saw a vision of this city in a dream and understood it to mean a message from God. After some private meetings with a delegation from Yathrib, at a time when the powerful in Makka made a concrete plan to assassinate Muhammad in his sleep, the Prophet migrated (“al-hijra”) to Yathrib with his close friend Abu-Bakr, skilfully evading bounty hunters sent after him. On arriving in Yathrib, he was cheered by the locals, who burst into song. Other followers from Makka later joined him and the new community of believers had found a peaceful new home. People bonded, renamed the city in the Prophet’s honour, and built a mosque together with a small house next to it for Muhammad. There he lived until he died.
The Prophet Muhammad would use the mosque to teach his believers (men and women together, it must be noted) about God and living morally righteous lives, as well as leading the entire city in civic matters. Migration or “hijra” to this new city proved so pivotal to the Prophet’s mission that, some 5 years after his death, it was chosen as the starting point for a new Islamic calendar - a calendar that is roughly 6 centuries newer than our conventional one.
There isn’t really a “happy new year” celebration in Muslim customs in the way that new years are celebrated. This is probably explained by another historical event within 50 years of Muhammad passing, that occurred early into the new year. The Prophet Muhammad’s grandson Hussain (or Husayn) was killed gruesomely in a battle for political power. The killing has special resonance with (the section of) “Shia” Muslims, and more broadly, the sobriety of the event has coloured the time of new year to take away its “happy” element. Indeed, because the start of the year is associated with sadness, marriages rarely take place during the (first Islamic calendar) month of Muharram in many Muslim societies.