CakeMaster said: Seems that way, but I'm not sure why.
Maitsuki Nanoka wa Rumia no Hi (basically 'The Seventh Day of Every Month is Rumia's Day') is a sort of celebration on pixiv that seems takes place every month on the seventh. As the name suggests, artists are encouraged to draw Rumia on that day! The date is a play on words, since "seven days" (七日) is pronounced "nanoka".
The 'happy birthday' in this one was directed to another artist on pixiv (who I don't think we have any work uploaded for.)