The seasons of the medieval year: What is Carnevale season?

The medieval calendar year was broken into different seasons with different tasks and responsibilities. The customs of each season evolved from ancient Roman customs, the growing and processing seasons of local crops, and the medieval Catholic Church year. 

The ritual year of the Catholic Church dominated most of Europe. The year was broken down like this:

Twelfth Night: Feast of the Epiphany or the arrival of the 3 kings to view the child. This is January 6th.
- This was the beginning of the Carnevale season, and the period where land was left fallow, and people were to huddle through the dark and cold of winter as best they could.
- This was the riotous celebration of the winter, associated with revelry and upending the usual structured social order. This aspect was probably a modernization of the Roman festival of Saturnalia and other Winter Solstice festivals.

Shrove Tuesday or Mardi Gras (=Fat Tuesday) was the last and wildest day of Carnevale season. It was the day before the obligatory 40 days of fasting and contemplation that was Lent.  For this reason, it was the last joyous or wild celebration before a a lengthy contemplation of one's sins and mortality.

Ash Wednesday: This was a day of fasting, penitence, and meditation 40 days before Easter. It began the period of Lent during which people were forbidden to eat meat and many other delicacies. Menus were leaner by religious custom and seasonal availability. It was a period of solemnity where people were supposed to deny themselves pleasures in order to ensure their place in heaven after they died.

The medieval calendar year began March 1, not January 1.

Easter: The big spring festival day of each year fell on the first Sunday after the first full moon after the spring equinox (very roughly March 21, but it had drifted due to a less exact medieval calendar. The Gregorian one we use was adopted in the Catholic world in 1583). There was an attempt to coordinate it with the Jewish feast of Passover. The Easter season continued until...

Whitsun (Pentecost): is the celebration of the day the Holy Ghost was supposed to have come to Jesus and his apostles. It fell on the 50th day after Easter, roughly around midsummer.

Corpus Christi (Feast of Christ the King): fell on the Sunday on or after November 20.

Advent: the  4 weeks before Christmas, a period of waiting and preparing for the dead of winter. The period when animals were slaughtered and farm buildings prepared to survive winter.

Christmas Day: Day the Christ Child was born (mythical, ofc) and not a big medieval celebration. Christmas celebrations lasted 12 days, however, until the Feast of the Epiphany (January 6).

For much of medieval Europe, the four "quarter days" where servants were payed and rents and contracts began were Lady Day (March 25, Feast of the Annunciation), Whitsunday or Midsummer Day (June 24), Michaelmas (September 29, Feast of St. Michael), and Christmas Day (December 25).

More on Saints Days and holidays in medieval and Renaissance Venice