Sforno on Exodus 34:21/ספורנו על שמות ל״ד:כ״א

1ששת ימים תעבוד וביום השביעי תשבות, success of your activities during the six days of the week also is materially affected by whether and how you observe the Sabbath.

2בחריש ובקציר תשבות, not only is Sabbath rest mandatory in the winter when the farmer has no urgent agricultural tasks, but also in the summer. Moreover, just as he will rest every seventh day he will rest every seventh year for the entire year. The Sh’mittah legislation applies both in summer and in winter. Both the weekly Sabbath, and the seven-yearly “Sabbath” have been called Sabbath by the Torah. (compare Leviticus 25,2) The success of our activities during the six years preceding the Sh’mittah year assumes that during the seventh year we will observe G’d’s law to abstain from agricultural activities.