The true names of the Mothers are Night & Day.
While Hecate has been a strong possibility (with textual evidence), I’ve always felt that that still isn’t ... fundamental enough for the Mothers’ true names. They are incredibly old & powerful, more elemental than a mere Greek goddess who was subordinate to Zeus - they are fundamental forces of nature without whom the world would be doomed.
Given the relevance of Ancient Greek mythology in the series as well as the WoJ about the Mothers which says they are close to angel level, I surmise that Mother Winter is Nyx - daughter of Chaos (the first being in Greek mythology), she is a being of primordial Night & is the mother of Hecate (in many traditions), the 3 Fates (Harry summoned Mother Winter calling her Atropos, one of these Fates) & many other beings like Thanatos etc.
In fact there is tale from Hesiod (most prevalent writer of Greek mythology in the BC era) where Day (Hemera, a daughter of Nyx) & Night (Nyx) live in the same house in Tartarus although they never occupy it at the same time.
