يَوْمَ الْحَسْرَ‌ةِ (the Day of Remorse) in verse 39 refers to the Day of Judgment and has been called the Day of Remorse because on that day persons consigned to Hell as punishment for their evil deeds will regret their disregard of the righteous way of life, and wish that if they had lived the life of true believers, they too would have been in Paradise instead of being punished in Hell. Even those who have been rewarded with Paradise will feel a different kind of remorse. Al-Tabarani and Abu Ya` la have quoted the following saying of the Prophet ﷺ on the authority of Sayyidna Mu'adh ؓ .

"The people of Paradise will regret only one thing, namely those moments which were spent without the remembrance of Allah."

And Al-Baghawi (رح) has narrated quoting Abu Hurairah ؓ that The Holy Prophet ﷺ said:

"Everyone will experience remorse and regret after death."

The companions of the Holy Prophet ﷺ asked him what would be that remorse and regret about? Then The Holy Prophet ﷺ explained that those who acted righteously will regret not having performed more righteous deeds which would have helped them attain a higher position in Paradise. On the other hand, the evil doers will repent their evil actions and sins. (Mazhari)