This happens in ep 9 11/08 - There's a moment coming up later this month between Luke and Lorelai that is so moving and poignant that it'll have Christopher seeing green. Two hints: said moment occurs outdoors, and it involves an infant. Source: Ask Ausiello @ TV Guide [Note: This could be for another episode.]
And in the 13 ep
I hear Episode 13 marks a turning point in the Luke/Lorelai/Christopher triangle. And Richard's health crisis plays a critical role. Source: Ask Ausiello @ TV Guide