The second flashback shows teenaged Lorelai preparing for her debutante ball. She is about three or four months pregnant, so it is perhaps April or May, and can no longer do up the zip on her debutante dress, which she was fitted for three months previously.
Lorelai hasn’t told her parents about her condition, and earlier told Rory that she still hadn’t told them on her sixteenth birthday, which is around April. This seems to be the moment leading up to Emily finding out about the pregnancy, although at first she blames the dressmaker for sending Lorelai a dress that is too small.
We know that Lorelai never got to go to debutante ball because she got pregnant, and this scene shows how it happened. Rory got to fulfil her mother’s role by making her debut, even wearing the same dress to her ball. The dresses don’t look the same – Lorelai’s has butterfly sleeves and Rory’s is sleeveless – but perhaps the dress was altered for Rory.
If Rory was born in October, she was most likely conceived in mid-to-late January, even though Lorelai and Christopher are shown kissing in December. It can perhaps be assumed that they continued having sex for a few weeks before conceiving Rory.
The Annotated Gilmore Girls 