Shabbat is mentioned several times in the Torah in relation to many key moments in the life of the Jewish people, including the creation of the world, the Exodus from Egypt, and the building of he Tabernacle.
Rav Shimshon Rafael Hirsch observes that in the parasha of Emor, Shabbat is placed at the head of the list of the yearly festivals, which are each described as opportunities to “to be callings of holiness” - to bring extra holiness into the world and into our lives, by means of the nation declaring each festive day to be holy.
Shabbat, which introduces the “holy convocations”, stands in contrast. Shabbat is not holy because the Jewish people declare it to be so; its unique sanctity is fixed by Hashem Himself. But precisely because it is an intrinsic source of holiness, it serves as the point of origin for the holiness that the Jews bring, through their own actions, to the other festivals.
It is Shabbat, with its message that Hashem is the ruler of nature (as demonstrated by creation) and history (demonstrated by the Exodus), that makes the acknowledgement of God a reality in our lives. And it is as a result of this weekly actualisation of the message, that the Jewish people are able to then imbue the other festivals with holiness. Thus Shabbat is truly, as we say each week in Kiddush, the beginning of the festivals, “of the callings of holiness”.