たぶん

Probably.

Sadly, the pun is lost in translation. The title literally just says, "nanoka nanoka". Nanoka (written 七日) can mean "Seventh Day of the Month" or "Seven Days", while Rumia's catchphrase, "is that so~?" is "so~nano ka~?" in Japanese. There, nano (written なの) is a feminine expression used to ask a question or make an assertion, and "ka" (か) is the particle that marks the previous statement as a question.

For this reason, the 7th day of each month is Rumia day in the Touhou fandom.