Here the Torah amplifies the function and ceremonial of the festivals that were first mentioned in Exodus. As before all special days are called “Moadim” occasions. And as before the first “holy Day” is Shabbat. Many Jews give priority to festivals over Shabbat. But in fact, in terms of priorities. It ought to be the other way round.
After Pesach comes the beginning of the harvest season. The first sheaf, the Omer, is brought to be dedicated the day after “the Shabbat.” Which we take to mean the day after the Seder Night of Pesach. The Ka’arites disputed this with the rabbis and claimed it meant the day after the Shabbat closest to Pesah. Here is an example of how the oral tradition determined the meanings of the Torah which were ambiguous or imprecise. The very fact that that the Torah lists Shabbat as the first of the Moadim can be interpreted in different ways.
Then the Torah goes on to declare that on the fiftieth day, after seven full weeks, there is a festival, a Mikra Kodesh, a Holy Gathering. But it does not give it a name. We now call it Shavuot, although the Torah says it is a day for Bikurim, First Fruits. But why doesn’t the Torah call it Shavuot here? And why does it not mention that it is also the anniversary of Sinai and Revelation?
Similarly, there is a reference to the festival in the Seventh Month that we now call Rosh Hashanah. It is simply referred to as a Shabbaton of Zichron Teruah, remembering the sound of the Shofar. Here too Oral tradition has interpreted and added on to what the Torah says.
It is therefore all but impossible to distinguish between the written and the Oral Law and to think that we can take one without the other. They are intertwined.