HA! Trick question. It's not:

Inrix, who supply data to in-car satellite navigation systems, reports that the worst mornings of the week – in the US, at least – are actually Tuesday, Wednesday and Thursday. Monday and Friday are low-congestion mornings. Friday evenings are bad, but Monday evenings are particularly quiet. My guess is that Monday is a popular day to take a holiday or call in sick.

But to be fair, it's not at all surprising that you might THINK traffic is worse then:

Norbert Schwarz – a psychologist who is well known to economists thanks to his work with Nobel laureate Daniel Kahneman – has famously showed that if you want people to tell you that their lives are a wreck, just ask them on a rainy day. There is also some psychological evidence that Monday mornings depress us, too.

In short, rainy days and Mondays always get you down. You’re living in a Carpenters song, but please don’t blame the traffic.

via timharford.com