Or HaChaim on Exodus 34:18/אור החיים על שמות ל״ד:י״ח

1 את חג המצות תשמור, "you are to observe the festival of unleavened bread." The reason that the Torah chose to mention this commandment at this point is the need for the Israelites to rehabilitate themselves from the sin of idolatry. The Zohar volume 4 page 40 states that the spiritual equivalent of leavened matter is an alien deity. This is the reason G'd has so insistently and repeatedly warned us about keeping or eating leavened matter on the festival of Passover. In this instance, G'd instructed us concerning the observance of all the festivals. The reason may be that all the major festivals are rooted in the Exodus experience of the Jewish people. Inasmuch as the sin of the golden calf elevated that idol to the redeemer which took the Israelites out of Egypt, all festivals are a reminder of the Exodus. G'd repeated the legislation because it contained elements which demonstrated that he who observed these commandments expressed his opposition to idolatry by doing so. He also mentioned Sabbath observance since the observance of the Sabbath counteracts the previously performed idolatry. Just as idolatry cancels all merits one ever accumulated by performing G'd's commandments, so the observance of the Sabbath is considered as observance of all the commandments and a form of rehabilitation for the idolatry one was guilty of previously. Compare my comments at the beginning of Parshat Vayakel.