The Kli Yakar asks why the Torah doesn't explicitly state that Rosh HaShanah is the Rosh HaShanah that we know. He says that the G-d didn't want us to mistakenly think that Teshuva and the related themes of Rosh HaShanah are a one day affair. Similarly, he says that the theme of Shavuot is left to the oral tradition so that the text of Torah does not allow us to think that accepting Torah, etc. is a once a year thing.