Here's the number of tweets including the phrase "slow news day," over the past three years.

May 18, 2012

Wow. Slow news day.
Wow. Slow news day.-- Jack Dorsey
We now know the Facebook IPO didn't go so well. So this was probably not a real slow news day, just not as big as Facebook might've hoped.
March 30, 2011

July 5, 2012

March 21, 2012

May 2, 2011

Slow news day.
Slow news day.-- Steve Martin
He got over 4,000 retweets. Given all the followup stories about the raid, May 2 wasn't a slow day for news, unless you compare it to the previous night.
So basically, days with a lot of "slow news day" activity on Twitter are either actual slow news days, or days when famous people are making jokes about slow news.