A week was a period of time equal to seven days. On Earth, the individual days of the week were respectively known as Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. Within the period of week was the weekend, which saw the end of one week and the beginning of another.
From the perspective of Tommy Brockless, who spent only one day awake per year after being taken to be cyrogenically frozen by Torchwood Three in 1918, only three weeks passed between the end of World War I and the beginning of World War II. (TV: To the Last Man)