What did Jesus do on the Monday of Holy Week?

He was arrested

Another Answer:

In Jewish tradition which is reflected in the Scripture, there are no days called 'Thursday' (5th day of week) which is now part of the modern Julian calendar.

That being said, Jesus just finished the Passover meal and was praying in the Garden when He was arrested. He was tried and crucified on the Passover daylight portion (Jewish days go from sunset to sunset unlike our traditional day).

Considering all the clues in Scripture, many scholars have determined that the Passover for the year 31 AD was on a Wednesday. This fully considers that Jesus ministered for 3 Passovers and was crucified on the 4th. He gave only one sign of His Messiahship - that of Jonah being 3 days and nights in the belly of the great fish. He arose just before sunset, exactly 72 hours after He was buried, and was seen in the darkness of the 1st day of the week - our tradition is Sunday - the day He ascended to the Father as the 'Wave Sheath' offering for all of mankind (pictured in the Feast of Unleavened Bread).

Recapping, He was crucified and buried on Wednesday, just before the start of the Feast of Unleavened Bread that began at sunset. Three nights and three days later, He arose on the Sabbath - Saturday. So on the whole of Thursday, He was in the tomb.