Sforno on Exodus 31:15/ספורנו על שמות ל״א:ט״ו

1ששת ימים יעשה מלאכה, during the six days of the week which correspond to the days when G’d created the universe you can busy yourselves with working on the construction of the Tabernacle. There is no good reason why this project should override the commandments governing the Sabbath. The only time a commandment overrides such commandments is when the Torah linked the commandment which would override it to a specific time frame, such as the circumcision of a baby on the eighth day after its birth, or the type of sacrificial service in the Tabernacle of which the Torah said that it must be performed every day i.e. including on the Sabbath. Any other commandment that is capable of being performed on a day other than the Sabbath does not override the Sabbath.

2שבת שבתון; this expression alludes to the fact that there are activities and even conversations and thoughts which, though not specifically outlawed by the text, are nevertheless something that interfere with the proper Sabbath observance. We have been told already in Exodus 23,12 וביום השביעי תשבות, that the cessation of work etc., has as its objective קדש לה', that this day is to be holy for the Lord, i.e. we are to make it holy. This means that on the Sabbath we ignore secular matters altogether and devote ourselves to spiritual pursuits, and even the mundane pursuits such as eating festive meals must be perceived as part of our honouring G’d on the Sabbath.

3כל העושה מלאכה...יומת. It follows that when someone performs secular tasks on the Sabbath he automatically loses track of the purpose of the Sabbath, hence he deserves to be executed.