Maybe a bit cheating - but it's nice with the consistency, and visually it becomes a lot more clear and immediate at least for me. "金" would be short for "金曜日"

Maybe a bit cheating - but it's nice with the consistency, and visually it becomes a lot more clear and immediate at least for me. "金" would be short for "金曜日", a common way to abbreviate weekday names in calendars etc and yes, Friday is indeed the day of gold - I like to imagine it's the old pay day for workers maybe? :D