Why do babies always get sick on the weekends?

So I came home on Friday and was looking forward to a fabulous weekend of romance. And then 

I heard, "I think she has a cough."

Oy.

Why do babies always seem to get sick on the weekends? Labor Day was the same. Cancelled all of our plans because of runny nose. But instead of wallowing in Web MD, we took advantage of the forced "at home" and enjoyed 3 days of alone time. Went to Costco and grabbed the big salmon and lots of discounted wine for a fun date night. And yes, our date nights are tending to end around 10pm unlike the wee hours of "before baby" (aka BB), but it's still date night!