We use the past simple to talk about actions and states which we see as completed in the past.
We can use it to talk about a specific point in time.
She came back last Friday.
I saw her in the street.
They didn't agree to the deal.
It can also be used to talk about a period of time.
She lived in Tokyo for seven years.
They were in London from Monday to Thursday of last week.
When I was living in New York, I went to all the art exhibitions I could.
You will often find the past simple used with time expressions such as these:
three weeks ago
from March to June
for a long time
for 6 weeks
in the 1980s
in the last century
in the past