45 Days of Lent?

I thought there were 40 days of Lent, not 45.  

Although, there are several Sundays, 6 to be exact, between today, the first day of Lent, and Easter.  Or does Lent actually start tomorrow?

But that wouldn't make sense either.  Why does Mardi Gras mark the last day of revelry before suddenly walking the straight and narrow?  Is Ash Wednesday just a go-between day?

I'm still confused though.  Even if you took Lent off on Sundays, that's 39 days of Lent.

What happened to 40?