I have been thinking you this all day.
On one hand, I agree with those who say you have every right to be cross and insist on your agreed on plans! (And what a pity this came on top of your other worries with your pet)
On the other, relationships often thrive on not always insisting on your "rights", in which case I would second those who are asking you to consider how close the friendship is, how you would like it to happen if it had been your friend etc.
Also how important if it to you are to celebrate that particular day. For me, like Ele, it wouldn't be a big deal to go on a different weekend (as long as it happened), but that's because my family often gives presents and has celebrations later.
I guess it depends a bit on your boyfriend has asked too! Has his request been done humbly, in a way that shows realizes how important this is to you, but could you change, or not?
Hopefully this situation will let you discuss things that help you learn more about each other and care for each other in a more informed way in the future.
And... happy 30th!!!