In Season 3, after Allison Cameron breaks up with Robert Chase for getting too emotionally involved in their "friends with benefits" relationship, Chase started I love you Tuesdays to remind her every week that he was still in love with her. Every Tuesday, Chase would remind Cameron of his feelings with a simple "It's Tuesday... I love you."
Cameron breaks it off after the events in Airborne. Chase makes it clear to her that he believes Cameron can't possibly be callous enough not to have an emotional attachment to him and won't make it easy for her.
In Act Your Age, Cameron is still angry with Chase, who points out she can't both break off the relationship and be angry with him. However, they come to an accommodation and, later in the episode, he gives her flowers.
In Family, Chase tells Cameron he likes her while they are performing tests in the pathology lab.
After Chase is fired in Human Error, Cameron meets him in a bar after work and he admits the whole idea was stupid. They part, but later that evening, Cameron comes by his apartment to say she wants to hear him say it again even though it's only Monday. Their relationship starts at that point.