He knew exactly how important it was as they'd both talked about the day and made joint plans for how to celebrate it.
He's now got what he thinks is a better offer with a mate, who he could see at any other time.
Why on earth should OP give up on their nice plans and pander to his grubby habit?
She's an equal member of this relationship and she's so upset about it she's posting on a forum. She's hoping someone will make excuses for his shit behaviour. I'm not going to do that, there aren't any.
She's not expecting him to be a mind reader, to surprise her on the day with roses and diamonds. She wants the day they had planned. She was honest with him about wanting a proper celebration and put a plan in place. How many times on here are posters asked why, when an occasion matters to them, they didn't explicitly say what they wanted. OP has done that and he's let her down completely.
People stay in crappy, unhappy relationships, with crappy, uncaring, irresponsible partners, because other people make excuses for them. Not sure why they're "boys and girls". OP is a grown up woman and her DP is a grown up man. He knows he's letting her down.
OP, you're worth more. Unless he's doing this to put you off the scent and has something amazing planned to sweep you off your feet (and that would be annoying anyway as you've made plans), he's being a dick.