Down in summer because you want the warmer air up high to be displaced by the cooler air below, thus, creating a cooling effect.
Up in winter because you want the cooler air down low to be displaced by the warmer air above, thus, creating a heating effect.
An easy way to remember is by 'alphabetical order'. D S U W - Down Summer Up Winter. (For the same info, see Fan FAQs.) My best to ya and hope this helps.
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