What had been called into emergency session during the summer in To Kill a Mockingbird?

Atticus was gone because the state legislature was called into emergency session. 

Atticus is a lawyer and a state representative.  His children are used to him working long hours, but sometimes when the legislature is in session he is away for weeks at a time.  Scout’s description of the legislative session tells us much about the historical events that were happening at the time. 

As if that were not enough, the state legislature was called into emergency session and Atticus left us for two weeks. The Governor was eager to scrape a few barnacles off the ship of state; there were sit-down strikes in Birmingham; bread lines in the cities grew longer, people in the country grew poorer. (Ch. 12) 

When she says “scrape a few barnacles off the ship of state,” that means trimming down government spending.  The sit-down strikes in Birmingham are related to the Civil Rights Movement, which was beginning to take root.  At the same time, the Great Depression has resulted in lengthening bread lines as people are desperate for aid.  The farmers were some of the hardest hit as their land turned into a dust bowl. 

Scout comments that none of these events really affected her.  Maycomb is insular.  She lives a comfortable life, because her father is a professional and has a nice house.  Even when he’s gone, Calpurnia the housekeeper keeps the house running.  Although she misses her father, summer is a time of exploration and hanging out with her brother Jem and Dill, her "permanent fiancé." 

Scout also describes a cartoon depicting Atticus at the legislative session. 

We were surprised one morning to see a cartoon in the Montgomery Advertiser above the caption, “Maycomb’s Finch.” It showed Atticus barefooted and in short pants, chained to a desk: he was diligently writing on a slate while some frivolous looking girls yelled, “Yoo-hoo!” at him. (Ch. 12) 

Jem tells Scout that it’s a compliment, because their father “spends his time doin‘ things that wouldn’t get done if nobody did ’em.”  He says that reorganizing the tax law is boring to most, but not to Atticus.  It’s symbolic.  Atticus does things no one else can or will do, such as defending Tom Robinson.