Sayyiduna Ibn Umar رضى الله تعالى عنه narrates: Verily some Sahabah رضى الله تعالى عنهم were shown in a dream that Laylatul Quadr is in the last seven days of Ramadhan, thus the Messenger of Allah (peace be upon him) said: I see that your dreams coincide, regarding the last seven nights, therefore one who seeks it (Laylatul Quadr), should seek it, in the last seven nights. (Bukhari/Muslim)
The last seven nights, refer to the 23rd night till the end of the month, according to most scholars.