Now is the time
The best time to study for a test is before you have to take it.
The second best time (notwithstanding), is after you've taken it the first time and failed.
I say failed, but what I really mean is 'tried and didn't get the result you were hoping for.'
That's because, fortunately for us, most of life's trials and tribulations aren't a once in lifetime test of mental, emotional or physical preparation.
And, outside of school, test-scores (of all kinds) don't matter much.
The point is, sure, yesterday was the best time to plant a tree. But if you somehow missed that memo, or forgot, today is as good as ever a time to start.