In commanding the mitzvah of sefiras haomer, the Torah says that we should begin counting "from the morrow of Shabbos" (Vayikra 23:15). The Talmud says that this on the morrow of the first day of Passover. Inasmuch as yom tov is a rest day, it is referred to as Shabbos. However, the Sadducees took this verse literally, that sefiras haomer must begin on Sunday, and Shavuos must always be on Sunday.
Bnei Yisaschar asks, "What is the point of the Torah referring to the first day of Passover as 'Shabbos,' thereby giving the Sadducees the option of misinterpreting it. Why did the Torah not simply say 'on the morrow of Passover? "
Bnei Yisaschar explains the difference between Shabbos and the festivals. The festivals occur on a particular day of the month - e.g. Passover on the 15th of Nissan. Inasmuch as the calendar is determined by the Sanhedrin on the appearance of the new moon, the kedusha of the festivals is essentially dependent on the act of the Sanhedrin. Not so Shabbos, which occurs on the 7th day of the week, independent of the Sanhedrin. This is why we say, Baruch Ata Hashem, Mekadesh haShabbos, that Hashem sanctifies the Shabbos, whereas on the festivals we say Baruch Ata Hashem, Mekades Yisrael veHazemanim. Hashem sanctified Israel (i.e. the Sanhedrin), who, in turn, sanctified the festivals.
Passover was unique among the festivals, because the Jews of the exodus were not deserving of the revelation of Hashem. The angels said to Hashem, "In what way are the Israelites better than the Egyptians? They are both idolatrous." Yet, Hashem revealed Himself to them, as we say in the Haggada, "with great awe refers to the revelation of the Shechina." Thus, Passover was as unique as Shabbos, receiving a kedusha from Hashem. To indicate this, the Torah refers to Passover as "Shabbos."
Shavuos, too, was an extraordinary Divine revelation. When we say that Shavuos is zeman mattan Toraseinu, it is not only in the historic sense. We can experience mattan Torah today as our ancestors did then.
In medicine there is a "recall phenomenon." An infant is immunized with several injections, causing the body to build up a huge quantity of antibodies to the virus. Over a period of time, the antibodies disappear from the blood stream, so that years later, their presence is virtually undetectable. If, many years later, the person is given a "booster" injection, the body promptly produces a massive amount of antibodies, just as with the initial immunization. The body "recalls" the earlier experience and reproduces it.
So it is with the intense spirituality of mattan Torah. Even if we are not at a lofty level of spirituality, with proper observance of Shavuos, we can have a "recall phenomenon," re-experiencing the extraordinary spirituality of our ancestors' declaration of naaseh venishma.
Rav Shlomo Walbe in Alei Shur says that we should use our powers of imagery, to see ourselves at the foot of Sinai, seeing the mountain ablaze and trembling, hearing the thunder and shofar, seeing Moses standing atop the mountain, and hearing the voice of Hashem saying, "I am the Lord, your G-d." In this way, we can have a "recall phenomenon." Zeman mattan Toraseinu can refer to a current experience rather than only to a historic one.