Allah descends every night to the lowest heaven when one-third of the night remains. The descent of Allah to the lowest heaven is in a real sense, in a manner that befits His Majesty and Might. Allah descends however He wills, whenever He wills.

It was narrated by Al-Bukhari in his Sahih (1145) and by Muslim (1261) from Abu Hurayrah (may Allah be pleased with him), that the Prophet (peace and blessings of Allah be upon him) said:

“The Lord descends every night to the lowest heaven when one-third of the night remains and says: ‘Who will call upon Me, that I may answer Him? Who will ask of Me, that I may give him? Who will seek My forgiveness, that I may forgive him?’”

It should be noted that the descent of the Lord – may He be Exalted – to the lowest heaven is one of the actions that are attributed to Him and has to do with His will and wisdom. It is descending in a real sense, in a manner that befits His Majesty and Might. Allah descends however He wills, whenever He wills. Glory be to Him, there is nothing like unto Him and He is the All-Hearing, All-Seeing.

It is not permissible to distort the meaning of the Hadith by saying that what is meant is that His command or mercy descends, or that one of His angels descends.

And Allah knows best

Source: IslamQA