Simon feels jittery after reading that Blue likes to imagine him fantasizing about sex. He's confused, mostly because Blue never writes stuff like that, and because Simon thought he was the only one who thought about the two of them having sex. He fantasizes about kissing Blue, who he thinks is Cal Price, and masturbates before he goes to bed.
Simon's confusion is, again, rooted in his beliefs as to what kind of a person he is and what kind of a person he assumed Blue was. His fantasies show clearly that his confusion doesn't mean he's not open to this kind of change, though it doesn't make it any less strange for him.
Monday is Simon's birthday. Leah grabs him in the morning, gives him a mix CD, and Simon apologizes for ditching her to go to the homecoming game. At lunch, Leah brings in a sheet cake, per tradition. Everyone is wearing party hats, as one must wear a hat to get a piece. Simon is surprised when Abby tapes a gold bowtie to his chest as Leah cuts the cake. The cake is perfect, and Simon thinks that Leah is amazing at celebrating birthdays.
Here, the bowtie from Abby represents yet another deviation from tradition, though not one that's as overt as some of the other changes Simon has noticed. This again makes it clear that as Simon grows up, plenty of things in his life are going to change in all sorts of ways, both big and small.
Simon takes the leftover cake to rehearsal. He sits on the risers to watch when he's not onstage, and Cal joins him to wish him a happy birthday. Nervous, Simon chooses not to tell Cal that his birthday plans are reading birthday wishes on Facebook and hanging out with his family, but does mention that he's getting an Oreo cake. Cal doesn't have any notable reaction to the mention of Oreos, but he does put his hand on Simon's shoulder as he gets up to leave.
Remember that Simon is pretty convinced that Cal is Blue; however, Cal's non-reaction to the mention of Oreos would suggest otherwise. Simon's nervousness around Cal suggests that he very much wants Cal to be Blue, which though Simon doesn't realize it, means that he's making a lot of assumptions about who Blue is based on relatively flimsy evidence.