Charles Ellicott Commentary Numbers 10:11

"And it came to pass in the second year, in the second month, on the twentieth day of the month, that the cloud was taken up from over the tabernacle of the testimony." — Numbers 10:11 (ASV)

On the twentieth day of the second month. —It appears from Exodus 19:1 that the Israelites encamped before Mount Sinai in the third month of the preceding year, and, as is generally supposed, on the first day of the month. In this case the encampment at the foot of Mount Sinai had lasted eleven months and nineteen days. No day of the month, however, is specified in Exodus 19:1, and no certain reliance can be placed upon the Jewish tradition that the Law was delivered fifty days after the Exodus. There is the same omission of the day of the month in Numbers 9:1; Numbers 20:1.