Other forms: weeks
Seven days make up one week, and there are 52 weeks in a year. If you won't see your friend again until next Thursday, you can say, "See you next week!"
Before the adoption of the Roman calendar, a week was more of a vague kind of change or succession, like the turning of a full moon to a half moon, for example. The modern week is strictly calendar-related, and seven days long, unless you're talking about a "work week" or a "school week," as in "I have Friday off, so my school week ends tomorrow!"