Dating somebody with the same birthday
So we feel like it is very rare to meet anyone with the same birthday as our own.
When you put 20 people in a room, however, the thing that changes is the fact that each of the 20 people is now asking each of the other 19 people about their birthdays.
If your date doesn't have plans, you can offer to take him or her out for a birthday drink, but be wary of this.
Sure, it's possible they're just not big birthday people, in which case it's perfectly fine to spend the day together without making a huge production out of the birthday.
Each individual person only has a small (less than 5%) chance of success, but each person is trying it 19 times. If you want to calculate the exact probability, one way to look at it is like this.
Let's say you have a big wall calendar with all 365 days on it. The next person who walks in has only a 364 possible open days available, so the probability of the two dates not colliding is 364/365.
It is likely that two people in the group will have the same birthday. The reason this is so surprising is because we are used to comparing our particular birthdays with others.
For example, if you meet someone randomly and ask him what his birthday is, the chance of the two of you having the same birthday is only 1/365 (0.27%).
Be sure to mingle with people, and don't expect to spend the whole night by the birthday person's side.
Of course, it's lovely if you do—but if you don't feel you're quite yet at the Birthday Dinner With Friends level yet, you can offer instead to take him or her out for a birthday drink on your own (possibly at a later date).
If you don't participate in a group celebration, some sort of acknowledgement about the birthday is still necessary.
The next person has only 363 open days, so the probability of not colliding is 363/365.
You consider breaking up with them, just to get out of the whole ordeal—maybe they would take you back in a week, once their birthday has passed? If You've Been Dating, Non-Exclusively, for a Few Weeks At this point, besides the fact that you guys have sex, your relationship isn't so different from that of good friends.