... (13 weeks in a season, just like cards in a suit) ...
"It was the Thursday of the Knave of Spring... "
(Which begs a question about day names...
OK, before the supremacy of the Christian church, all European, Middle Eastern, and Asian cultures called the days after the names of the "planets" as regarded by ancient astronomers/astrologers... Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. And the planets were named after gods, so northern cultures had Sun, Moon, Tiw/Tyr, Woden/Odin, Thunor/Thor, Frega/Freya, and (strangely) Saturn ... although in Nordic countries Saturday is "Bathday"!
[There's an excellent article on Wikipedia: Days of the week
As the Hyborian Age is of this Earth, the planets known to sages then would be the same as ours, just given different names. I don't recall REH mentioning any by name, however.
But you could easily divise appropriate names for the different Hyborian cultures.)
Which tangential musing suggests taking an astrological basis for the mont h names in a Hyborian campaign... after the signs of the Zodiac, perhaps, as it was for the Babylonians. (Including Ophiucus, the snake-fighter or healer, between Scorpio and Sagittarius if you want a calendar of 13 months, although the Zodiac is really a solar not a lunar concept.) See: Zodiac