Lent is 40 days of preparation before Easter, akin to Jesus’ 40 days of fasting in the desert – everyone knows that, right?
The truth is actually a bit more complicated.
Most people get 40 days by counting the days from Ash Wednesday through Holy Saturday (the day before Easter), which gets you 46 days, then removing the 6 Sundays to get 40 days. The idea is that Sundays are “days off” from the penance of Lent and so don’t count toward the total.
There are two major problems with this theory.
First, Lent ends on Holy Thursday, not Holy Saturday, since the last three days before Easter are their own separate season called the Paschal Triduum. That gets you something like 37 days if you still don’t count the Sundays.
Second, the Sundays are a part of Lent. They are even called just that: “The second Sunday of Lent,” for example. If you include them in your count, you get to 43 days. But as we saw above, even if you don’t include them, you still don’t get to 40 days.
The truth is that the idea “Lent is 40 days” is just approximate. It’s around 40 days.
Another source of the confusion is that the precise practices surrounding Lent have changed over time. The idea that there was a 40 day period of penance prior to Easter dates back to the early Church, but things have gotten a bit more complicated over time with the introduction of Ash Wednesday at the beginning of Lent and the Triduum as a separate season.
[See also: 16 Hilarious Memes to Kick Off Your Lent]