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You will also hear it's best as far in advance as you can, best as close when they have tickets they can't sell, better on Tuesdays, better on Sundays, better on Saturdays, better at night, etc.
There's no way to know when the best time is to get tickets. Some sites like Bing offer a fare predictor suggestion to buy now or wait, but I've never found it to be very accurate, and it isn't always able to make suggestions.