Hi, you would usually just say 'one day last week' As in 'It was one day last week' or 'I'll go shopping one day next week'. but... If you specifically wanted to say that something happened once, for example if someone asked you 'how many days in the last week did you go to the shops?' you could answer 'one day in the last week' or 'twice in the last week'. I hope that helps :)

What is the context?

I would just say "One day last week". For example, "One day last week, I realized that I haven't washed my hair in 4 days!" Silly example sentence I guess :-)