Ember Days this September: 18, 20 and 21
From New Advent:
Ember days (corruption from Lat. Quatuor Tempora, four times) are the days at the beginning of the seasons ordered by the Church as days of fast and abstinence. They were definitely arranged and prescribed for the entire Church by Pope Gregory VII (1073-1085) for the Wednesday, Friday, and Saturday after 13 December (S. Lucia), after Ash Wednesday, after Whitsunday, and after 14 September (Exaltation of the Cross). The purpose of their introduction, besides the general one intended by all prayer and fasting, was to thank God for the gifts of nature, to teach men to make use of them in moderation, and to assist the needy. The immediate occasion was the practice of the heathens of Rome. The Romans were originally given to agriculture, and their native gods belonged to the same class.
At the beginning of the time for seeding and harvesting religious ceremonies were performed to implore the help of their deities: in June for a bountiful harvest, in September for a rich vintage, and in December for the seeding; hence their feriae sementivae, feriae messis, and feri vindimiales. The Church, when converting heathen nations, has always tried to sanctify any practices which could be utilized for a good purpose. At first the Church in Rome had fasts in June, September, and December; the exact days were not fixed but were announced by the priests. The "Liber Pontificalis" ascribes to Pope Callistus (217-222) a law ordering: the fast, but probably it is older. Leo the Great (440-461) considers it an Apostolic institution. When the fourth season was added cannot be ascertained, but Gelasius (492-496) speaks of all four. This pope also permitted the conferring of priesthood and deaconship on the Saturdays of ember week--these were formerly given only at Easter.
Before Gelasius the ember days were known only in Rome, but after his time their observance spread. They were brought into England by St. Augustine; into Gaul and Germany by the Carlovingians. Spain adopted them with the Roman Liturgy in the eleventh century. They were introduced by St. Charles Borromeo into Milan. The Eastern Church does not know them. The present Roman Missal, in the formulary for the Ember days, retains in part the old practice of lessons from Scripture in addition to the ordinary two: for the Wednesdays three, for the Saturdays six, and seven for the Saturday in December. Some of these lessons contain promises of a bountiful harvest for those that serve God.
From Catholic Culture:
Since man is both a spiritual and physical being, the Church provides for the needs of man in his everyday life. The Church's liturgy and feasts in many areas reflect the four seasons of the year (spring, summer, fall and winter). The months of August, September, October and November are part of the harvest season, and as Christians we recall God's constant protection over his people and give thanksgiving for the year's harvest.
The September Ember Days were particularly focused on the end of the harvest season and thanksgiving to God for the season. Ember Days were three days (Wednesday, Friday and Saturday) set aside by the Church for prayer, fasting and almsgiving at the beginning of each of the four seasons of the year. The ember days fell after December 13, the feast of St. Lucy (winter), after the First Sunday of Lent (spring), after Pentecost Sunday (summer), and after September 14 , the feast of the Exaltation of the Holy Cross (fall). These weeks are known as the quattor tempora, the "four seasons."
Since the late 5th century, the Ember Days were also the preferred dates for ordination of priests. So during these times the Church had a threefold focus: (1) sanctifying each new season by turning to God through prayer, fasting and almsgiving; (2) giving thanks to God for the various harvests of each season; and (3) praying for the newly ordained and for future vocations to the priesthood and religious life.