Short answer
At sunset on the 20th day of Ramadan. Because the Islamic night comes before its day, the first of the last ten nights is the night before the 21st fast. The odd nights in which to seek Laylatul Qadr are the nights before the 21st, 23rd, 25th, 27th and 29th days.
The last ten begin at sunset at the end of the 20th day. The sunnah itikaf starts by entering the masjid before sunset on the 20th and ends when the moon of Shawwal is seen. If Ramadan has 29 days, the last ten are nine nights and the itikaf is still complete.
The four schools agree on this.
Explanation
In the Islamic calendar the day begins at sunset, so each night belongs to the day after it. The 21st night is the night before the 21st fast. So the last ten nights begin at Maghrib on the 20th day of Ramadan, when you break your 20th fast. See why the Islamic day starts at sunset.
The odd nights. The Prophet ﷺ said to seek Laylatul Qadr in the odd nights of the last ten. These are:
- 21st night: the evening of the 20th day.
- 23rd night: the evening of the 22nd day.
- 25th night: the evening of the 24th day.
- 27th night: the evening of the 26th day.
- 29th night: the evening of the 28th day.
If your community began Ramadan on a different day from another, count from your own first fast. The Laylatul Qadr tonight page works this out for your city.
What to do. Aishah said that when the last ten began, the Prophet ﷺ would tighten his waist wrap, stay up the night and wake his family. Men who can should consider the sunnah itikaf: enter the masjid before sunset on the 20th and stay until the Eid moon is seen. See the itikaf guide and how to pray on Laylatul Qadr.
If Ramadan has 29 days, the last ten are nine nights, and their virtue is not reduced. The four schools agree on how the nights are counted.
The evidence
Quran, Surah Al-Qadr 97:3
لَيْلَةُ ٱلْقَدْرِ خَيْرٌۭ مِّنْ أَلْفِ شَهْرٍۢ ﴿٣﴾
The Night of Decree is better than a thousand months.
The last ten nights hold Laylatul Qadr.
Sahih al-Bukhari 2017
Sahih... تحروا ليلة القدر في الوتر من العشر الأواخر من رمضان ...
"Seek Laylatul Qadr in the odd nights of the last ten of Ramadan."
The odd nights of the last ten are the nights to seek it.
Sahih al-Bukhari 2024
Sahih... إذا دخل العشر شد مئزره وأحيا ليله وأيقظ أهله ...
"When the last ten began, he tightened his waist wrap, gave life to his nights and woke his family."
The last ten nights deserve extra worship.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on itikaf. The sunnah itikaf of the last ten begins by entering the masjid before sunset on the 20th.
- al-Hidaya, Kitab al-Sawm, chapter on itikaf. Itikaf in the last ten days of Ramadan is sunnah.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.