Four sets of three days that mark the turn of the seasons.
They fall in the four seasons: after the first Sunday of Lent, after Pentecost, after the Exaltation of the Cross in September, and after Saint Lucy in December.
Each set is a Wednesday, a Friday, and a Saturday, kept with fasting, prayer, and thanksgiving for the fruits of the earth.
They were traditionally the days on which men were ordained, and remain a fitting time to pray for priests.
The Ember days tie the liturgical year to the actual year: seed, harvest, vintage, and the dead of winter. They are a corrective to a faith that floats free of the ground it is lived on.
The quattuor tempora are Roman and very old, likely Christianizing older agricultural observances of the city, and they were universal in the Western calendar for centuries.
The 1969 reform left their observance to national conferences rather than fixing them in the general calendar, so in most countries they quietly disappeared. They were never abolished.
Not obligatory in most places today. Keeping them privately is a good and thoroughly Roman thing to do.