Jamieson, Fausset and Brown on Exodus 31:12-17

12-17. Verily my sabbaths ye shall keep--The reason for the fresh inculcation of the fourth commandment at this particular period was, that the great ardor and eagerness, with which all classes betook themselves to the construction of the tabernacle, exposed them to the temptation of encroaching on the sanctity of the appointed day of rest. They might suppose that the erection of the tabernacle was a sacred work, and that it would be a high merit, an acceptable tribute, to prosecute the undertaking without the interruption of a day's repose; and therefore the caution here given, at the commencement of the undertaking, was a seasonable admonition.