I really can't grasp the idea... So the context, there are 2 characters let's call them Hanako and Yosuke. Yosuke congratulates Hanako because it's her birthday and Hanako says "ありがとうよすけ" then the following line (in the picture) seems to belong to Yosuke. I can't even understand well what does 君がしてくれる means here, if another verb in て form would have been before it, I could have understand differently... Then I can only understand something about a date, but I can't form a connection properly, I am really stumped. Understood the rest of the comic, except this part. Any interpretation would be useful, maybe then I can form a better connection.
君がしてくれるみたいに ＋ 日付が変わったらすぐお祝いしてあげたかった
I wanted to congratulate you as soon as the date changed + like you do for me
She's saying that he celebrates (perhaps by sending him a text) her birthday at right as the clock hits midnight and the next day starts, and that she wishes she could have done the same for him. The whole thing is in past tense presumably because she did not manage to.
Breaking it down a little more:
君がしてくれるみたいにliterally "like you do for me".
日付が変わったらすぐliterally "soon when the date changes", but more natural as "as soon as the date changes". Also see here.
お祝いしてあげたかったliterally "wanted to celebrate for you", though probably more natural as "wanted to congratulate you"