You are right that the first day of Great Lent is Monday, February 23 (as calculated by the Revised [New] Julian Calendar, which happens to coincide for the next few centuries with the Gregorian Calendar). However, the fortieth day is Friday, April 3. We count Sundays, but not Holy Week, for which we fast of course as well. You have to remember that Great Lent does not mean "this is when we fast". It is a liturgical period - note that many services are done quite differently during those forty days. Holy Week services are separate. It took me a few years to figure this out, too. You're not alone.