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Ideally start on a Monday. But there is no need to go back and work on ones you have already done. In the beginning you are not expected to complete many of the Daily Challenges. Spend 10+ mins on it, mark it complete, and then move on to the next challenge the next day. Progress over perfection is the motto.
The Daily Challenges repeat, so the first time through you might only be able to do the first line or maybe only the first measure. The next time through you will get further, by the 3rd or 4th time you will have spent enough time playing guitar that you can probably get through the entire challenge.
So try your best but don’t try to perfect it, it will come around again.