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I've seen suggestions that it's better to practice a month prior to a contest, rather than at any other time prior to the contest.

What is special about a that time frame that it would be suggested so often?

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Sunspots significantly alter propagation characteristics, and the sun revolves at a 26 to 38 day rate. Since it's gaseous, it revolves differentially - faster at the equator and more slowly at the poles.

While individual sunspots can last from days to weeks, sunspot activity on a position on the sun changes slowly over time. While you can practice at any time prior to the contest the sunspot activity will be most similar about 28 days prior to the contest, and thus propagation conditions will be most similar.

28 days works well because sunspots most frequently appear just north and south of the equator, in the bands with the most turbulence, and it fits pretty well with the 7 day week we humans tend to follow.

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