English for Today by Ganiu Abisoye Bamgbose (Dr GAB)
DAY (NOT, DAYS)
Back in the ~days~, we had friends in all parts of the country (nonstandard).
Back in the day, we had friends in all parts of the country (standard). The expression is used to talk about a time in the past, usually when you are remembering nice things about that time.
In their ~heydays~, they sold more records than all other labels in the country (nonstandard).
In their ~hey days~, they sold more records than all other labels in the country (nonstandard).
In their heyday, they sold more records than all other labels in the country (standard). Someone’s heyday is the time when they are most powerful, successful, or popular.
GAB