At the risk of over-simplifying, here goes:
Lent begins on Pure Monday (two days earlier than Ash Wednesday). However, the pre-lent period begins two weeks earlier with Meatfare week (we say farewell to meat) followed by Cheesefare week (we say farewell to dairy products. “Meat” by the way includes fish–that is fish “with a backbone” so shellfish such as shrimp is allowed. The fast consists of both fast (when we eat) and abstinence (what we eat). Usually a strict fast (only water) is observed from midnight till noon. But there is no limit (other than avoiding gluttony, of course) on the amount of food from noon to midnight. The lenten practice is observed Monday through Friday, but not on Saturday (the Sabbath) or Sunday (which is always a celebration of the Resurrection), and is mitigated on the Feast of the Annunciation whatever day of the week it may be. Wait! It gets more interesting. Lent is over by Palm Sunday. So with the two pre-lent weeks plus the six weeks of Lent proper there are 40 days. 5 x 8 = 40. But another fast is observed for Holy Week.
This would be the strictest observance. A more moderate observance would be the strict fast on Wednesdays and Fridays (or Mondays, Wednesdays, and Fridays) and having some dairy or fish on Tuesdays and Thursdays. On Saturdays and Sundays, you still use restraint. The idea is not to go on a binge, but moderate use of dairy and fish is permitted.
Another interesting tweak to Byzantine practice is that fake foods (meat substitutes, non-dairy creamer, etc.) are not considered to be “in the spirit of the fast.” In other words, that’s cheating! An example of the proper use of “fake foods” is if you always use non-dairy creamer, then it is proper to use it during lent. If you normally use half & half in your coffee, then it is “cheating” to use non-dairy creamer. Same thing goes for soy “meats.”
As you can see, this is not a simple question–or at least not a question with a simple answer.
One other thing, Great Lent is only one of four fast periods in the year. The others are: (1) the pre-Christmas fast (also called the Philippian Fast because it begins on the Feast of St. Philipp November 14). (2) the Apostles’ Fast (from the day after the Feast of All Saints until the Feast of Sts Peter and Paul). In the Eastern calendar All Saints is on the Sunday after Pentecost. Because the date of Pentecost varies, this fast can be as long as six weeks or as short as a few days. (3) the Dormition fast is the 14 days before the Feast of the Dormition (Falling asleep of the Virgin Mary) which is called the Assumption in the western church, so that is from August 1 to August 14 with mitigation for the Feast of the Transfiguration.
In addition, a strict observance would include abstaining from worldly entertainments (e.g., movies) and taking on extra spiritual reading, additional prayer, etc.
Now this is the part that will shock the Latins: Because the Eucharist is a celebration of the resurrection in Eastern theology, the weekdays of lent are a-liturgical. That is, the Divine Liturgy (“mass”) is not celebrated. However, because we need “food for the journey,” we have pre-sanctified liturgy on Wednesdays and Fridays of Lent and during Holy Week. Pre-sanctified liturgy is vespers with a communion service. The bread has been consecrated during the Sunday Divine Liturgy and is given to the faithful on Wednesday and Friday.
Well, now that I have practically written a book, I’m sure others will have more to add.
