Disclaimer: I don't really get birthdays but evidently lots of people do, so this is just my take on it, I respect that others will have very different views on it!
WhyTF is your 'big birthday' such a big deal? I mean, it's a big deal to you, but like weddings and subsequent anniversaries surely the only people who care are the people involved?
I got married. I've had two anniversaries. Dh and I swapped cards and some small presents but I was a bit
at other people sending us Happy Anniversary cards... its very nice but I can't believe some people remember this shit.
Re birthdays it happens every year. Unless someone has been incredibly unwell and overcome the illness then I don't know what is so amazing (for everyone else) about it being another year since you were born.
I mean it's nice for people to think of you and send a card or maybe a present but people who are suggesting they should rearrange their dream wedding because it's someone's birthday...? If you had 100 guests at a wedding there could be any number of birthdays or other significant anniversaries to consider...
It certainly isn't as exciting or important imo as someone's wedding.
As pp have said reply and say you were only joking, can't wait to see them on their special day and then on your 'big day' you'll be all dressed up, and you can just spend the day drinking and eating lovely food and drink probably for free and will probably have access to some professional photos of it all too... 
What a lovely birthday!
On another some PPs said
I think people are often encouraged to suck up bad behaviour, in order to avoid conflict, which is how other people get away with behaving like twats, so much of the time
I so agree with this, you see it on here and in real life so much. "Dont say or do anything - it will cause a family row" type thing.
This is mind blowing for me. I have been accepting what I consider to be bad behaviour in the name of not rocking the boat. People can indeed be utter twats when permitted. 