Tafsiran Ulangan 10:5: Chizkuni, Or HaChaim, Tur HaArokh & Ramban

ואפן וארד מן ההר, “I turned around and descended from the Mountain;” on the same day, on Tuesday, the tenth day of Tishrey. This is what is written in Exodus 34,29: ויהי ברדת משה מן ההר, “it happened while Moses was on his descent from the Mountain, etc.;” even though Moses had delayed briefly on the Mountain, he descended on that day and brought the second set of Tablets with him. This day is not included in the third 40 days that Moses had spent on the Mountain. Those forty days had already been completed at dawn of the day that Moses descended. Moses’ absences from the people, and his fasting, therefore commenced on Monday, the first day of Sivan, prior to the revelation on the sixth day of the month, as he had to completely discharge any remnants of food and drink consumed before he ascended the Mountain where he had to be comparable to angels who do not eat or drink. He therefore could remain in a constant state of ritual purity. He completed his fasting on Monday the ninth day of Tishrey. This is why it became a custom among some of our people to observe Mondays and Thursdays commencing with the first day of Sivan and concluding on the tenth day of Tishrey.

כאשר צוני השם, "as G'd commanded me." Moses meant that all the time it took for him to bring the Tablets to the people and to place them in the ark constructed by Bezalel, the Tablets remained in the ark which he had constructed and taken up to Mount Sinai with him.

ואפן וארד מן ההר ואשים את הלוחות בארון אשר עשיתי, “I turned around and descended from the Mountain and placed the Tablets in the ark that I had constructed.” I did this immediately after descending from the Mountain .