Season 7. The two learned more about each other and started to grown on one another. Eventually the two comforted each other and slept together, not realizing in a few short months, Meg would be carrying a Nephalem, their unborn child, Cassandra Master.
But then, in 4.04 Metamorphosis , Dean says to Sam: Cas said that if I don't stop you, he will. That was the first time he ever calls Castiel 'Cas'. And I wonder if it's because he got a bit comfortable with Cas after 4.03 In The Beginning , when Cas laid a hand on his shoulder and gave him a sympathetic look.