Islam's holy book Qur'an was revealed to Prophet Mohammad in the month of Ramadan. This is a month of fasting and prayers for Muslims. And the last tens days are of great importance because it's believed that Qur'an was revealed during this time.
Laylat al-Qadr known by Muslims as the Night of Destiny.
This marks the anniversary of the night Muslims believe the Quran was revealed to the Islam’s prophet Muhammad, PBUH, in its entirety by angel Gabriel.
This night being of historical significance, Muslims are highly recommended to worship as much as they can on this night. It is also believed to be the night when God decides the destiny of everyone.
Among other customs for this night is reciting the holy Qur’an, Muslims continue these acts of devotion from night till early in the morning. Usually in mosques or religious centers, but some get crowded like the Arg mosque of Tehran which I was at and people continue this worship all around the mosque.
This Lailatul Qadr is to be found in the last nights of Ramadhan, mostly on the 19th, 21st or 23rd night of fasting month. For Shia muslims the occasion coincides with the night that their first Imam, Ali was attacked during prayers in the great mosque of Kufa, an injury that led to him passing away three days later.
For all Muslims these last days of Ramadan are of great importance and some call it the heart of the year as this being a turning point in the calendar.