As you well know, the Congress was not -scheduled- to reconvene until December. President Lincoln called them back early for a special session.
President Lincoln -expected- the rebellion to have collapsed by July.
He really did think that loyal Union men in the south would reassert themselves and depose the traitors. That is partly why he was willing to force a confrontation at Fort Sumter.
Walt
And as you well know, many in Congress rightfully believed that war was just around the corner and expected without any reason for doubt that they would be immediately reconvened if it broke out while they were away. In fact they spoke of this almost daily in the March 1861 special session of the senate.
War broke out not long after the Senate went home, in large part because that is when Lincoln started actively agitating over the southern forts. He got his war only two weeks later yet waited until July to bring Congress back. The only reasonable explanation is that he did not want Congress there to interfere while he was waging his war.