You can freeze unbaked spanakopita triangles, but we don’t recommend freezing them after they’ve been baked as they tend to become a bit soggy.  To do so, arrange them on a baking sheet, and place them in the freezer until hard. Transfer the triangles to a freezer-safe container and keep them stored for up to 2 months. When you’re ready to eat, bake as usual, adding extra baking time as needed until the dough is crisp.