When you told your husband that the festival, which was booked in January, clashed with his lads holiday, what was his reaction? A person who had genuinely forgotten the dates, would be mortified at their mistakes. They'd be apologising, seeing if they could rearrange the lads holiday or looking into the possibility of coming back early from it.
Your husband doesn't appear to be mortified, he hasn't apologised and hasn't tried to rearrange. In fact all he's done is tell you that you never want him to go on his lad's holiday.
To me, that's not a man who made a genuine mistake. I'd hazard a guess that those dates were the only dates all his friends could make. Instead of saying he couldn't go, because he didn't want to be the only one who couldn't and potentially miss out, he went along with it. He never checked the dates with you, because he knew he couldn't go. Instead, he's waited several weeks to tell you, because he's hoping you'll say 'Oh dear what a shame, don't worry dear, you go on your lads holiday, whilst I parent our 3 children on my own at a festival'.
You need to say to your husband, that he needs to decide who is more important, his family (his daughter who's birthday treat this is for) or his friends.