this day last week; a week ago today
Fixed expression meaning exactly one week before today. Used as a noun, often with particles like は or が.
This day last week was rainy.
Do you remember what you were doing a week ago today?
Refers to the day before yesterday, not a week ago.
Means 'last week' in general, not a specific day.
Compound of 先週 (last week) + の (possessive particle) + 今日 (today), literally 'last week's today'.