The Calendar lets you keep track of each week of in-game time. This is particularly useful for keeping track of when your Lifestyle's rent is due, your progress on Extended Tests, and what your character has been up to each week. This is setup to mimic the calendar used by the Shadowrun Missions. Each month has 4 weeks with every 3rd month instead having 5 weeks.
The first time you click the Add Week button, you'll be asked to enter a year, month, and week. This will be the starting point for your character's calendar. Each time you click Add Week after that, a new entry will be created for the next week, automatically rolling over to the next month and year.
You can edit the notes for any given week by either double-clicking on it or by selecting it and clicking the Edit Week button. When you're done entering notes, you can close the Notes window by pressing Esc or by clicking the red X.