['I usually take my brother to lunch for his birthday, but not this year. His birthday was yesterday.', "Why weren't you able to do it?", "I forgot about his birthday. I didn't even call him!", 'Oh man that sucks, do you feel bad about it?', 'Yes. Horrible!']
I'm sure if you apologize he'll understand at least a little. Were you just really busy or something?
Yesterday was my brother's birthday and I forgot to call him. I always call him on his birthday and we go to lunch. I feel so bad.