I don't get it.

11th of April; Rumia -- Youkai of Darkness; Desk drawer

Hmmm....

Yeah, I thought translating it might help explain the joke.
I was wrong.

The 7th (七日, pronounced "nanoka") of every month is Rumia Day.
I think Nanoka is rumia's catchpraise "is that so?" in japanese. not 100% sure on that one.

i think the joke here is rumia was 4 days late on her own special day of the month.

@Zekana: I don't think that was the joke.

Can someone please check the source on pixiv to get the context from the the artist's comment? Thanks.

Zekana said:
The 7th (七日, pronounced "nanoka") of every month is Rumia Day.

sweet that means my birthday is rumia day

makkun said:
@Zekana: I don't think that was the joke.

Can someone please check the source on pixiv to get the context from the the artist's comment? Thanks.

Actually, that is the joke. The title of the picture is, "The 7th of every month is what again ...?".