According to Holy Prophet (PBUH) : Try to find "Laiiltulqadar" in odd nights of last ashra of Ramadan.
But it is not necessary that 27th of Ramadan is "Laiiltulqadar" it may be 21st 23rd 25th 27th or 29th.
On 27th of Ramadan Nazool e Quran was completed so importance of this night is more so Reward on praying this night is equal to 80 years of "ibadah"thanks.
islam.stackexchange.com/users/32656/Sundas Shireen
Laylatul Qadr has many opinions on when it is like the night of the 27th of Ramadan which has a Hadith from Abu Dawud and it is from this Hadith that most scholars probably orient towards the 27th: Narrated
Mu'awiyah b. Abi Sufyan : The Prophet (sallallahu 'alaihi wa sallam) as saying:
Lailat al-qadr is the twenty-seventh night (of Ramadan) حَدَّثَنَا
عُبَيْدُ اللَّهِ
بْنُ مُعَاذٍ
، حَدَّثَنَا
أَبِي
، أَخْبَرَنَا شُعْبَةُ، عَنْ قَتَادَةَ
، أَنَّهُ
سَمِعَ مُطَرِّفًا
،
عَنْ مُعَاوِيَةَ
بْنِ أَبِي سُفْيَانَ
،
عَنِ
النَّبِيِّ
صلى الله عليه وسلم فِي
لَيْلَةِ الْقَدْرِ قَالَ ‏ "‏
لَيْلَةُ الْقَدْرِ لَيْلَةُ سَبْعٍ
وَعِشْرِينَ ‏"‏
‏.‏
And in Sahih Bukari , The Prophet tells us to seek it in the odd numbered last days of Ramadan, 21st, 23ed, 25th,27th, 29th: 'Aishah (May Allah be pleased with her) reported: The Messenger of Allah (sallallahu 'alaihi wa sallam) used to observe I'tikaf in the last ten days of Ramadan and say, "Seek Lailat-ul-Qadr (Night of Decree) in the odd nights out of the last ten nights of Ramadan."
[Al- Bukhari].
وعنها، رضي الله عنها، أن رسول الله صلى الله عليه وسلم قال‏:‏ ‏ "
‏تحروا ليلة القدر في الوتر من العشر الأواخر
من رمضان‏"‏ ‏(‏‏(‏رواه
البخاري‏)‏‏)‏‏.‏
It is also said that the Prophet used to stay up and do Ibadah when the ten last days of Ramadan came.
islam.stackexchange.com/users/9/مجاهد
When is Laylatul Qadr?
Narrated by Aishah (رضي الله عنها‎)
Mohammed (صلى الله عليه و آله وسلم‎) says, Search for the Night of Qadr in the odd nights of the last ten days of Ramadaan.
So it could be: 21 23 25 27 29
There is not fixed that laylatul qadr in 27 ramadan.
Source: Laylatul Qadr
islam.stackexchange.com/users/21983/Tafi Nami
I believe that a lot of scholars say that Laylatul Qadr is on the 27 night of Ramadan because the Quran was revealed on that night.
islam.stackexchange.com/users/34153/shifa
Basically the most narrations on the topic of laylatul Qar that seem to give hints on which night this special night may correspond say it is among the last ten nights of Ramadan or even restrict it on the odd nights among these last ten nights.
Note that a sahih narration of ibn 'Abbas (see here in Sahih al-Bukhari) even seem to disagree with the odd nights as it also suggests preparing for it on the 24th night.
How ever there are two statements that actually totally disagree with or exclude the 27th night being this special night,
one of them is that of ibn 'Abbas, Abu Sa'id al-Khudri and 'Ubadah ibn as-Saamit who gave the hint to seek it on the 21st, 23rd and 25th night of Ramadan (all these narrations appear in at least one of the two Sahih books) and another is that of 'Abduallah ibn Unais which concentrates on the 23rd night and this was compiled in Sahih Muslim.
Actually only the most soundest narration saying it is the 27th night of Ramadan is that of Ubay ibn Ka'ab which appears in Sahih Muslim : By Allah, I know about Lailat-ul Qadr
and I know it fully well that it is the twenty-seventh night (during Ramadan) on which the Messenger of Allah (ﷺ) commanded us to observe prayer.
(Shu'ba was in doubt about these words:" the night on which the Messenger of Allah [may peace be upon him] commanded us to observe the prayer.
" This has been transmitted to me by a friend of mine.)
Another sahih narration saying so is this: Salim reported on the authority of his father that a person saw Lailat-ul- Qadr on the 27th (of Ramadan).
Thereupon Allah's Apostle (ﷺ) said: I see that your dreams agree regarding the last ten (nights of Ramadan).
So seek it on an odd number (of these ten nights).
( Sahih Muslim )
Ibn Hajar al-'Asqalani commented the hadith of Mu'awiya which appears in Sunan abi Daowd in his Bulugh al-Maram : Mu'awiyah bin Abi Sufian (RAA) narrated, ’The Messenger of Allah (ﷺ) said regarding the night of al-Qadr, "It is the 27th night (of Ramadan).’’
Related by Abu Dawud.
But it is most probably the saying of Mu’awiyah and not the Prophet (ﷺ) .
Then he referred the reader to his book Fath al-Bary where he listed 46 opinions on the special nights and added the most likely among all of these (according to him)
is that it is among the odd nights among the ten last nights of Ramadan and that this night changes from year to year and the most likely view of the majority is that it is the 27th night of Ramadan.
وأرجحها كلها أنها في وتر من العشر الأخير، وأنها تنتقل، وأرجاها عند الجمهور ليلة سبع وعشرين For further references and details
please refer to How many times or when do we need to make the 'laylatul qadr dua'?
where I've listed many of those narrations.
islam.stackexchange.com/users/13438/Medi1Saif
From most of the scholars I've heard that Laylatul Qadr could be on any of 21st, 23rd, 25th, 27th and 29th of Ramadan. But why do they orient it toward 27th?
Why is Laylatul Qadr commonly thought to be the 27th night of Ramadan?
islam.stackexchange.com
<ramadan><laylat-al-qadr>
islam.stackexchange.com/questions/1227
islam.stackexchange.com/users/59/Tabrez Ahmed
It is not perfectly clear why Laylatul Qadr is thought to be the 27th night of Ramadan, but one possible explanation is that it was the night that the Quran was revealed or completed. In  Sahih Bukari , The Prophet tells us to seek it in the odd nights out of the last ten nights of Ramadan. There is also a Hadith  from Abu Dawud that encourages most scholars to probably orient towards the 27th. In addition, a lot of scholars say that Laylatul Qadr is on the 27 night of Ramadan because the Qur'an was revealed on that night.
Many scholars say that Laylatul Qadr is on the 27 night of Ramadan because the Quran was revealed on that night. Laylatul Qadr may commonly be thought to be the 27th night of Ramadan because scholars believed that is supposedly the night that the Quran was revealed.
A lot of scholars say that Laylatul Qadr is on the 27 night of Ramadan because the Qur'an was revealed on that night. There is also a Hadith  from Abu Dawud that encourages most scholars to probably orient towards the 27th. Also, in  Sahih Bukari , The Prophet tells us to seek it in the odd nights out of the last ten nights of Ramadan. Basically most narrations on the topic of laylatul Qadr seem to hint at the last ten nights of Ramadan or even restrict it on the odd nights among these last ten nights.
Laylatul Qadr may commonly be thought to be the 27th night of Ramadan because scholars believed that is supposedly the night that the Quran was revealed.