when do i plant holly oak seedlings?

i find the evergreen oaks more difficult than deciduous

When may not be as important as how. I find that oaks tend to come up on their own timetable, and the older they are when transplanted, the more shock they experience. Now how I plant them, I make sure that they have some mycorrhizal fungi growing with them as best I can add it. I collect mushrooms growing at the base of oaks (best time is after a heavy rain) and put them in the blender with some water. Use this for an acorn soak, drench potting soil with it, pour it in the hole you dig for the seedling, and pour it on the mulch you put around the tree. You can't overdo. When acorns sprout into media that is full of fungal spores, of a type that is symbiotic with oaks, then the seedlings do very well.

Oaks are quite dependent on mycorrhizal fungi, and most members of the Bolete family will form commensal associations with most oak species. Now if you manage to get some truffles to grow with your oaks, then you will have it made.