I might be just thinking too much into it, but I don't think Nezumi would really like the idea of birthdays? For him, it would be just a date that doesn't mean much. Also, I don't think the Forest People would have the same calendar as us. My theory is that the Forest People don't even have names, and that's why Nezumi was first named by Rou, so going by that theory, I wouldn't think they really have the concept of birthdays either. But again, it's just my way of viewing the culture of the Forest People, it's not like it's official or anything ^^"