Luke: He makes sure the day is all about you - and becomes kind of a slave for you. He cooks, or at least attempts to cook, your food, takes you out to your, do everything you love, showers you with gifts - even though you insisted saying it wasn't necessary. He'd remind everyone it was your birthday - either it was from a picture of you both together with the capitol 'It's my babe's birthday! Everyone go wish her a happy birthday 'cause I love her to the moon and back!', or just dedicating a to you at one of his show.
Ashton: Birthday sex. Lots of it. It was your birthday after all, it was his pleasure to pleasure you. Ashton would dedicate every second of that day to make you feel special, loved, sexy... From the moment you'd up, he'd be already waiting for you, whispering "are you ready for your first present?", and then taking you your breakfast in bed. Only to go with you to "help" you with your shower. At randoms moments, he'd hand you a present, saying that he thought of you when he saw it and had to buy it - and judging by the amount of presents you'd receive, he'd think of you a lot.
Michael: The for your birthday would begin some months before the actual date. Michael would start to pay a lot more attention to every little thing you'd say, slowly planning a perfect date for the both of you on the special day. You'd wake up to an empty bed with a note of his pillow, something only you and him you'd understand; somewhere only you'd both know. There, you'd find another hint of where to go and, hint by hint, you'd find him, waiting with everything you'd need for your special day out. Completely thought, planned and made by him.
Calum: He'd reserve the day for the both of you, spending it whole inside your house. Both of you wearing comfortable clothes - or none at all -, cooking together, talking, watching some movies, and you receiving your gifts. But the greatest of them would be at night, when you'd go to the party he had organized for you. You knew he was planning it, of course you did, he himself would tell you that. But you didn't know where it would be, who would be there and the special details. And that's what made it even more perfect.