the especial night mentioned in surah Dakan means which night.is it the shaban or Qadar?

Laylat al-Nusf min Sha’baan (the
15^th of Sha’baan) is like any other night, and
there is no sound report from the Prophet
(peace and blessings of
Allaah be upon him) to indicate that on this night the fate
or destiny of people is decided. 

See Question no. 8907. 

With regard to the night mentioned in the
verses: 

“We sent it (this Qur’aan) down on a
blessed night. Verily, We are ever warning [mankind that Our
Torment will reach those who disbelieve in Our Oneness of
Lordship and in Our Oneness of worship].

Therein (that night) is decreed every
matter of ordainments”

[al-Dukhaan 44:3-4] 

Ibn Jareer al-Tabari (may Allaah have mercy on
him) said: the commentators differed concerning that night,
i.e., which night of the year it is. Some of them said that
it is Laylat al-Qadr, and it was narrated from Qurtaadah that
it is Laylat al-Qadr. Others said that it is the night of the
15^th of Sha’baan. The correct view is the view of
those who say that it is Laylat al-Qadr, because Allaah has
told us of that when He says, ‘Verily, We are ever
warning’ [al-Dukhaan 44:3].”

(Tafseer al-Tabari,
11/221) 

With regard to the phrase, “Therein (that
night) is decreed every matter of ordainments”: Ibn Hajar
said in his commentary on Saheeh al-Bukhaari: “What
this means is that the decrees for the year are decided on
that night, because Allaah says: ‘Therein (that night) is
decreed every matter of ordainments’. And al-Nawawi said:
the scholars said that it is called Laylat al-Qadr because on
this night the angels write down the decrees (aqdaar),
because Allaah says, ‘Therein (that night) is decreed
every matter of ordainments’. This was also narrated by
‘Abd al-Razzaaq and other mufassireen with saheeh isnaads
from Mujaahid, ‘Ikrimah, Qutaadah and others. Al-Toorbashti
said that the word used in the Qur’aan is qadr, whereas
people think it is qadar (decree); the word qadr refers to
the detailed manifestation of the decree. 

Laylat al-Qadr brings great reward for the one
who does good deeds and strives in worship on that
night. 

Allaah says (interpretation of the
meaning):

“Verily, We have sent it (this Qur’aan)
down in the Night of Al-Qadr (Decree).

And what will make you know what the Night
of Al-Qadr (Decree) is?

The Night of Al-Qadr (Decree) is better
than a thousand months (i.e. worshipping Allaah in that night
is better than worshipping Him a thousand months, i.e. 83
years and 4 months).

Therein descend the angels and the Rooh
[Jibreel (Gabriel)] by Allaah’s Permission with all
Decrees,

(All that night), there is peace (and
goodness from Allaah to His believing slaves) until the
appearance of dawn”[al-Qadr 97:1-5] 

There are many ahaadeeth which speak of the
virtue of that night, such as the hadeeth narrated by
al-Bukhaari from Abu Hurayrah, in which the Prophet
(peace and blessings of
Allaah be upon him) said: “Whoever spends Laylat al-Qadr in
prayer out of faith and hoping for reward, all his previous
sins will be forgiven, and whoever fasts Ramadaan out of
faith and hoping for reward, all his previous sins will be
forgiven.” (Narrated by al-Bukhaari, al-Sawm,
1768).

And Allaah knows best.