One amazing source is the log books of Rudy Van Gelder, who was as fastidious with information as he was with sound. Every session is documented and has been a great research tool for me.
ECM albums rarely were made in a day, which is why they tend to list month and year. That said, when a single session date was in effect, they rarely included that. One example that comes immediately to mind is Arild Andersen's A Molde Concert, which just says August 1981 but took place on August 4th according to the Molde Jazz Festival people I contacted.
One early label that was great about including recording dates was Argo.