These fasts are no longer of precept, of course, but that doesn't mean that their use wouldn't be spiritually fruitful.
This applies particularly in Lent. The Western Lenten fast is as follows:
The Lenten Fast
All weekdays of Lent are days of fasting and abstinence. That means one single meal. Two lighter meals may be taken as long as their combined quantity does not exceed that of the single meal.
Meat may not be eaten, nor, I understand, fish, though I may be corrected on this. I presume it (and remember reading it somewhere, but I can't find it) on account of fish being specified as permitted on Sunday.
Traditionally, the abstinence also forbids eggs and all dairy products, the so-called 'black fast'. (perhaps because of the milkless tea). This had ceased to be of obligation by the nineteenth century.
In practice, this means observing a vegan diet during the week.
Oil may be used (unlike in the east) to cook or dress food at all times.
Sundays in Lent are days of abstinence, but not fasting. Therefore the normal quantities of food may be eaten, but not meat. Fish is permitted.
Fasting and abstinence are only lifted should the day be a Holy Day of Obligation. I don't think any holy days would fall within Lent these days.
Days of Fasting outside Lent
In addition, Wednesdays, Fridays and Saturdays during Ember weeks are days of fasting. Ember weeks are the first week in Lent, (fasting anyway), the Octave of Pentecost, The third week in September and the third week in Advent.
The vigils of the following feasts are days of fasting: Pentecost, Ss Peter & Paul, the Assumption, All Saints, and Christmas Day.
All Wednesdays and Fridays in Advent are fast days.
Should a fast day fall on a Sunday, it is observed on the Saturday. Should a feast fall on a Monday, the fast is also observed on the Saturday.
The only exception to the Friday abstinence traditionally was if Christmas day should fall on a Friday.
