I'm expecting company tomorrow and wish to serve it hot, sparing prepping tomorrow.
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You can freeze spanikopita before baking and then just add 10 minutes or so to the baking time. The end product is as delicious as baking as soon as it's made.
Further to your question about refrigerating, if you refrigerate, the mix will permeate the filo and end up soggy without crisp, distinct layers around the mix. The only way to avoid this is bake right after preparing or freeze and bake just before serving.
My wife's mother and yia yia both would freeze for up to even six months. When baked it tasted as if she made it that day. Don't refrigerate though, unless you want soggy spani.