Wk. is the short written form of week. You will see it where space is tight and nobody wants to write the whole word: a calendar square, a timetable, a work schedule, a note to yourself. "2 wks." means two weeks; "wk. 3" means week three of a course or a plan. It is writing only — when you read it out loud, you say the full word "week" or "weeks", never the letters. Two things worth knowing. First, the dot at the end is what marks it as a shortening, and British writers often drop it and just write wk, while American writers tend to keep wk. — both are fine, and nobody will correct you. Second, wk. belongs to the informal, practical corner of writing. In a proper letter, an essay, or anything you hand to a teacher or a boss, write week out in full. Save the short form for your own notes and for forms and tables where the space really is small.