The Church officially states that Lent is from Ash Wednesday until Holy Thursday before the Mass of the Last Supper. That’s 43 1/2 days. If you were to subtract the Sundays, you’d get to 37 1/2, a few days short. The other “solution” to the problem is to say all of Holy Week is not part of Lent. That also takes off 6 days, but does so in a way that the 2 1/2 days are part of the 6 taken off. That is also incorret because the Church states that Lent lasts until Holy Thursday. So the reality is that Lent is symbolically 40 days but really 43 1/2 days long.

The history of people not considering the Sundays part of lent has to do with penance and fasting. For very good reasons, many people broke their Lenten fasts on Sundays as a way to ensure that they celebrate the Resurrection. This idea was then extended to mean that “Sundays weren’t part of Lent”. But that is an overstatement. We can start and stop any Lenten fast (minus the handful of manditory days) whenever we wish because they a matter of personal devotion, not Church law. Our decisions to break the fasts on Sundays is not an indication that Sunday’s are not part of Lent but that we also choose to honor the celebration of the Resurrection, even though it is Lent.

See http://www.jimmyakin.org/2004/03/the_length_of_l.html for a more detailed explination of Lent and the Church documents to support it.