like: ngày đầu → →
on the first day ☊ 
trong ngày đầu tiên ☊ New Year's Day ☊ day ☊
the early days of computers ☊ day ☊
in the early days of space exploration (= when it was just beginning) ☊ early ☊
I've got three days' holiday including New Year's Day. ☊ including ☊
I don't remember my first day at school. ☊ remember ☊