Macarons get better as they age. So filling them in advance and leaving them in the fridge actually makes them better. See for example this:
"Macarons, against all pastry traditions, actually get better with age. The shells soften and become more chewy, mingling with the flavor of the buttercream too. So, while of course you can eat them right away, don’t hesitate to store them refrigerated for up to a week."
Source: http://bravetart.com/recipes/Macarons
Definitely fill them the day of. Not sure where you live but if it's humid I would make them the day of. If it's dry, making them a day in advance is ok.
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"Macarons, against all pastry traditions, actually get better with age. The shells soften and become more chewy, mingling with the flavor of the buttercream too. So, while of course you can eat them right away, don’t hesitate to store them refrigerated for up to a week."
Source: http://bravetart.com/recipes/Macarons