The j/k keys come from the Vi text editor and ADM3A computer terminals, where they originally were used for vertical navigation – they mean "cursor down" and "cursor up" respectively. See HJKL keys for the full set.
(Nowadays you'll also find these hidden hotkeys in various other webapps, such as Twitter timelines or Ars Technica comment threads.)
Because j moves the cursor down in this keymap, it makes sense for it to scroll down to the next item – and it used to be an obvious choice in earlier Google Calendar UI versions, which used vertical scrolling for the month view. (If I remember correctly, that is.)
The only problem is that the "Material 2" redesign for Google Calendar switched to sideways scrolling for months, which no longer matches the hotkey choice. (And, IMHO, is completely unintuitive for a calendar in general – even when I'm swiping left/right on the Android app.)