(1) a day after; after a day = 一天后/经过一天
(a) He died a day after his wife left him.
(b) After a day staying in hospital, he died peacefully.
(2) a few days ago = 几天前(谷歌翻译错误:日前)
(a) He died a fews days ago
(b) His wife left him a few days ago. He died yesterday.