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Hi Bob ~ I suppose atmospheric conditions mainly

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Leaving atmospheric conditions to one side...The biggest, brightest occurs when the moon is at perigree & is also 'full'. In recent times this was December 12th 2008 when it was 28,000km closer than average to the Earth

 The next encounter with a moon as close & as full as 2008 will not be until November 14th 2016

A perigree moon that's full appears 14 per cent bigger & 30 per cent brighter than most full moons

There is also the phenomenon known as the Moon illusion which is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. But I would suppose that the brightest moon on a given day will be when it is as its highest because the light takes its shortest path through the atmosphere to your eye then

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Michael

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