Roland learned from Anna that Tilly had her twentieth birthday coming.
He hadn't thought about such a thing until now.
It wasn't because he was careless, but because Prince Roland's memories were like a database that he would only delve into when necessary. However, his memories rarely contained anything related to politics or occultism and mostly consisted of mischievous wrongdoings and ideas for pranks, so he revisited them less and less.
Moreover, Tilly was a sore spot for Prince Roland, so everything related to her had little significance and had been buried away, as if he didn't want to recall them.
Tilly's birthday didn't mean her Day of Awakening, but her actual day of birth. As an Extraordinary, she didn't know the time when she awoke as a witch, nor would she experience the magic bite during the Months of Demons. Therefore, her birthday was her only important milestone.