Usually during the last 10 days of Ramadan but it varies by year and religious sect
About Laylat Al-Qadr
Laylat Al-Qadr (Also known as Shab-e Qadr) is also known as the Night of Destiny. It is celebrated by Muslims during the Month of Ramadan – the most holy month of the year. It celebrates the day the Koran was revealed to the prophet Mohammed by Allah.