What is Isra and Mi'raj?

The Isra: Mecca to Jerusalem

The journey is mentioned at the opening of Surah al-Isra.

The Prophet ﷺ described riding al-Buraq, a white animal larger than a donkey and smaller than a mule, to Jerusalem, where he prayed two rak'ahs in the mosque (Sahih Muslim 162). Muslim tradition holds that he led the earlier prophets in prayer there.

The Mi'raj: through the heavens

From Jerusalem, the angel Gabriel ascended with him through the seven heavens. He met prophets along the way, including Adam, Idris, Moses, Jesus and Abraham, and reached the Lote Tree of the utmost boundary (Sahih al-Bukhari 349).

Then came the gift of prayer. Allah first prescribed fifty prayers a day. On Moses' advice, the Prophet ﷺ went back and asked for a reduction several times, until Allah said these are five prayers equal to fifty in reward (Sahih al-Bukhari 349). So every time you pray, you're doing something given directly in that meeting. See Do I have to pray five times a day right after becoming Muslim?

Body and soul, or a vision?

The majority of scholars hold that the journey happened in body and soul, while awake, which is why it was such a test of faith for the Quraysh. A minority of early voices described it as a spiritual journey. When the Meccans mocked it, Abu Bakr believed it immediately, which is how he earned the title al-Siddiq (the truthful). The story also shows the unity of the prophets. See Do I have to believe in all the prophets to become Muslim?

Is the 27th of Rajab a holiday?

Many communities hold talks on the 27th of Rajab to retell the story, and it's a public holiday in some Muslim countries. Historians disagree on the exact date, and there is no authentic special prayer or fast set for that night. Scholars differ on gatherings: many see a talk about the story as a good reminder, while others avoid fixing a yearly celebration. Learning the story is good on any day. See How does the Islamic calendar work?