In this Parsha, we find a listing of all the holidays and many of the unique Mitzvos associated with them (ויקרא פרק כ"ג). At the beginning of this section, however, we find once again the commandment to keep Shabbos. The introductory Posuk of the section says "מועדי ה' אשר תקראו אתם מקראי קדש," "these are the festivals of Hashem which you shall declare as holidays" (שם פסוק ב'). But the next Posuk says "וביום השביעי שבת שבתון," "on the seventh day is Shabbos" (שם פסוק ג'). If the purpose of this section is to present the laws of the holidays, why does it begin with a reference to Shabbos? Rashi (לפסוק ג' שם) raises this question and explains that the connection between Shabbos and Yom Tov is drawn in order to illustrate that if one desecrates a Yom Tov, it is similar to desecrating Shabbos, while if one keeps the laws of Yom Tov, it is similar to keeping Shabbos. In other words, Shabbos is mentioned because the people were already familiar with its laws and its significance; Hashem wanted them to understand that the laws of the Yomim Tovim, which are being presented here, are of similar importance and significance.
Rav Moshe Feinstein, in his Sefer דרש משה, expands upon this connection between Yom Tov and Shabbos in the following way. One of the purposes of Shabbos, he explains, is to enable us to show our belief that Hashem created the world. Therefore, if one violates Shabbos, it is as if he denies that Hashem created the world. Yom Tov, on the other hand, reminds us that Hashem still controls the world, because the Yomim Tovim all enable us to focus on events which demonstrate Hashem's continued involvement in the world, long after He created it. As observant Jews, we can not believe in Hashem only partially, by saying, for example, that we believe that Hashem created the world, but that He then gave it over to "nature." Nor can we believe that Hashem now runs the world but that He did not really create the world Himself. If one would hold either of these positions, there would be no reason to fulfill the words of the Torah, because such belief in Hashem is incomplete. Therefore, when one desecrates Yom Tov, it is as if he has desecrated Shabbos, because his observance of Shabbos demonstrates only a partial belief in Hashem; he shows by observing only Shabbos, and not Yom Tov, that his belief in Hashem is incomplete and it is therefore not particularly significant. Similarly, if one keeps Yom Tov but not Shabbos, this observance would also be relatively meaningless, because this person too is demonstrating only a partial, and thus incomplete, belief in Hashem. It is for this reason that we start speaking about the Yomim Tovim in our Parsha by mentioning Shabbos, because both messages are required in order to teach a Jew to demonstrate his complete faith in Hashem.
We must all take this message about Shabbos to heart and realize what a great opportunity Hashem has given us with this great gift which allows us to declare our belief in Him on a weekly basis. We should thus realize that we must give Shabbos the utmost respect, and keep it in our minds even during the week, as we count the days of the week towards Shabbos, and as we prepare our best food for Shabbos, as described by the Gemara in Beitzah (דף ט"ז.). Although one may think that the expenses incurred to observe Shabbos properly are overwhelming, one should bear in mind the statement of the Gemara in Shabbos (דף קי"ט.) which indicates that certain Jews deserved to become wealthy specifically because they honored Shabbos properly. This may mean that because these people went out of their way to honor Shabbos and thereby declared their belief in Hashem, Hashem rewarded them for their complete faith. These same ideas should apply to observing Yom Tov as well because, as noted above, the observance of Yom Tov complements the observance of Shabbos; the Gemara in Beitzah )שם( indeed speaks about spending money to honor Yom Tov as well as to honor Shabbos. If we look at the observance of Shabbos and Yom Tov as demonstrations of faith in Hashem, we will perhaps be inspired to work harder on our observance of these days. The Gemara in Shabbos (דף קי"ח:) states that if the Jewish people can observe Shabbos two times according to the law, we will be immediately redeemed. Perhaps this is because Hashem will then see the kind of faith we have in Him and He will respond by treating us favorably. May this be fulfilled speedily in our days.