How sound is the attribution to Ibn Al-Qayyim, may Allah have mercy on him, of the saying: "From sunset on Thursday to sunset on Friday, every second within it holds treasures of good deeds; so let us increase our prayers upon the Prophet, peace and blessings be upon him"?

We did not find this wording attributed to Ibn al-Qayyim. What he mentioned regarding the virtue of sending blessings upon the Prophet (peace be upon him) on Friday and its eve is what he stated in Zād al-Maʿād: "It is recommended to frequently send blessings upon the Prophet (peace be upon him) on this day and its eve, due to his (peace be upon him) saying: 'Increase your sending of blessings upon me on Friday and the night of Friday.'"

Sending blessings upon him on this day has a special distinction, because every good that his Ummah attained in this world and the hereafter was attained through him. And Friday is a day of Eid for them in this world, and on it, Allah grants them their requests. So, out of gratitude to him and fulfilling a small part of his right, we should increase our sending of blessings upon him on this day and its eve.