One of the Parashot HaMoadim, a discussion of the Chagim throughout the year, is located in this week’s Parashah. It is interesting to note that although this section opens with “Eileh Heim Moadai,” “These are my appointed festivals” (VaYikra 23:2) the next Pasuk states, “Sheishet Yamim TeiAseh Melacha UVaYom HaShevii Shabbat” “For a six-day period Melacha may be done, and the seventh day is Shabbat.” (23:3) Rashi asks, if the section is supposed to be dealing with the Mo’adim, why does it open with Shabbat, which is not a festival? He answers that this comes to teach us that if someone violates Yom Tov, it is as if they violate Shabbat.
However, there seems to be a further problem with this connection of Shabbat and Yom Tov. Yom Tov is decided based on the sanctification of the New Moon by Beit Din, but Shabbat is based on the weekly calendar and occurs without any human intervention. The Gemara (Rosh HaShanah 24a) helps answer this by stating that Hashem declares a new month in Beit Din Shel Maalah (heavenly court) only on the last possible day, while the Beit Din on Earth can declare the beginning of a new month beforehand and establish it for themselves and the Beit Din Shel Maalah. So by connecting Shabbat to Yom Tov, the Torah is reminding us that just as Hashem has predetermined for us when Shabbat comes, so too we determine for Him when Yom Tov occurs.
A third explanation is given by Rav Yonatan Eibeshutz. He states that Shabbat opens the section of the Moadim to teach a Halachah. When Beit Din declares Rosh Chodesh, sometimes it will cause them to override some Halachot of Shabbat. For example, extra sacrifices known as Mussafim may need to be brought on Shabbat, or on Shabbat Rosh HaShanah, the Shofar may need to be blown in the Beit HaMikdash. By opening the section with Shabbat, the Torah is teaching us that Beit Din’s declaration of the New Moon even has the power to override Shabbat.