The day in the Menstrual Cycle when a girl becomes most fertile, and is usually very near to Ovulation. Because of increased hormones she becomes extremely horny, and greatly desires sex.
You can tell if a girl is on Day 11 if she repeatedly mentions dirty or sexual things or just wants to have incredible amounts of sex. Be careful on this and the following days though, she's horny for a reason; it's babymakin time and the oven is warm.
Cassie: I called 8 dudes in my contact list, 4 of which aren't even booty call status and none of them will come and fuck me. AHHHH!!!
Jeff: Are you that easy or is it Day 11?
Cassie: Day 11
Chris: DUDE! Me and Stacy went for 7 rounds today. I couldn't believe it, we both called in for work sick and romped all day long!
Steve: Gotta love Day 11, I hope she's on the pill!