The initial part of this parasha characterises aspects of holiness of the priests,and is then followed by a general description of the holy days of the Jewish year. Ramban interpreted the phrase “mikra’ay kodesh” (holy convocations) as implying the duty of all Israel to gather in the Shule to celebrate the Chagim with prayers and hymns to the glory of G-d. Rashi interpreted the connection between “mikra’ay” and “kara”(call) in the sense of calling these days as a time of celebration, and R’ Yitzchak Meir of Ger explained that “mikra’ay kodesh” meant calls to holiness.
Despite being about the Chagim, the chapter begins with Shabbat, after six days of work, a day of complete rest. R’ E Munk connected this precedence given to Shabbat with the sequence in the holiday prayers “mekadesh hashabbat, Israel ve’hazmanim”. He wrote that G-d sanctifies the Shabbat which in turn passes its holiness to Israel which in turn can consecrate the Chagim.
The Chagim have a dual character. R’ Munk wrote that from the view-point of history, they remind us of events of great importance in our past, and from the view-point of nature, they connect with the yearly cycle of the changing seasons. Pesach marks the beginning of the harvest, Shavuot comes at the end of the harvest, and Sukkot after the gathering of the produce from the threshing floor and wine press. The historical events are tied to the lunar calendar, and the agricultural events are synchronised with the seasons and solar calendar.
The Sanhedrin managed the calendar adjustments until after Israel’s exile from the land. The permanent calendar was instituted by Hillel the younger in 360 CE. The extra day was instituted by the Sanhedrin to deal with doubts about the sighting of the new moon, and was kept by Hillel as a reminder of the spontaneity with which we should celebrate these special days of communion with G-d.