It was during winter when they were fifteen, obviously before Andrea found out of her feelings for Bree. Becca decided to throw a small Christmas party, inviting some friends and their kids as well so that her kids could make some new friends, well not technically her kids, just Andrea and Bree. Prior to this, she was informed that Andrea and Bree were showing signs of difficulty in socializing, but little did they know that not socializing was just laziness for Andrea and nonchalance for Bree.
Obviously on the wrong track, Becca threw the party. And when the guests started to arrive, Bree was nowhere to be found. Becca couldn't leave her guest so she told Lucas and Andrea to go look for her. Lucas knew exactly where she would be and when he told Andrea that Bree would be in the back lake house, Andrew was skeptical but when they did find Bree in the lake house, her skepticism turned into utter jealousy.
