The birthday of Johan and Nina has always been a topic of speculation. Although the manga marks the year as 1975, the day or month has remained a mystery, generally thought to fall around May.
The images above illustrate a conversation Nina has with her father after seeing some children wearing costumes earlier in the day. He explains that their outfits are in celebration of Saint Adrian, a Herculian Guard who was martyred on March 4, 306 after becoming a Christian convert.
The Heidelberg arc takes place over the span of seven days, meaning the twins’ birthday falls on March 10, 1975.
A more detailed analysis can be found here.














