If you trust him, you could explain to him that you inadvertently found out when you were checking out your options for a therapist. It seems clear to me that you were not stalking him and he should understand the need to check out your options before you choose a therapist. It might be helpful to discuss with him what you told yourself about having a difficult session on his birthday, as it might help you to understand your process, better your therapeutic relationship, and work toward your goals.

I was in the same boat as you - found out my therapist's birthday without meaning to when I checked out his resume online.(And his birthay is the day after mine, too!) While as a therapist I assume responsibility for when I work and when I don't and when I decide to work on my birthday I don't expect or want clients to spare me, when I know it's my therapist's birthday on the day of our session or even just a little before I also avoid difficult subjects. But I never avoid talking about my negativity - I just try not to give him very negative feedback on his birthday That said, I feel very special when he works with me on his birthday. So I understand this may bring up feelings and thoughts of going easier on a therapist if you know this. But if he is working on that day, he is probably expecting that his clients (including you) may bring some really heavy stuff. Now that you've canceled, I wouldn't worry about it.