Although the Children of Israel went out of Egypt and became a nation in Nissan - the first month, most of the Festivals are celebrated in Tishrei – the seventh month. In the seventh month, both the historical aspect - the end of the process of the Exodus, as well as the cosmological aspect - the beginning of the agricultural year are commemorated.
The first month in the Jewish calendar is Nissan, because of its historical significance, being the month in which we went out of Egypt and the start of our history as a nation. Yet, most of the Festivals are celebrated in Tishrei, the seventh month of counting following the Exodus. Tishrei contains four Festivals, one per week – Rosh Hashana, Yom Kippur, Sukkot and Shmini Atzeret. The connection between the sanctity of the seventh month and the Festivals celebrated in it is emphasized by the repeated use of the term 'sabbatical' to refer to the Festivals of Tishrei, a term not used with the other Festivals.
The special status of the month of Tishrei comes from the centrality of the number seven in Judaism. The seventh is sanctified – the seventh day is sanctified as the day of rest before God, the seventh year is sanctified as rest for the land and the seventh month is also a sanctified month.
Comparing the seventh month with Shabbat and the Shmitta year seems strange at first glance. Shabbat commemorates the end of the week, Shmitta is the end of the agricultural cycle but in contrast, the seventh month comes in the middle of the twelve-month year. This difficulty can be explained by the cyclical nature of the year. The furthermost point away from the beginning of the year is exactly in the middle – on the first day of the seventh month. The seventh month is the furthest away from the first month, in which we went out of Egypt and thus represents the end of the process of the Exodus.
An additional reason for the special status of Tishrei is because the world was created in this month, according to the majority opinion of our Sages. In the Land of Israel, the first rain of the season falls during the month of Tishrei, reminiscent of the momentous event when mist fell upon the earth for the first time and quenched its thirst: "And a mist ascended from the earth and watered the entire surface of the ground." (Gen. 2:6, Rosh Hashana 11a).
From the viewpoint of Jewish history, Tishrei represents the end of the Exodus, and yet from the viewpoint of the universe, it represents the beginning. These two aspects of the month are also expressed in the content of the Festivals.