In a large bowl, combine the olive oil, lemon juice, lemon zest, garlic powder, salt, and black pepper. Whisk until well combined.
Add the sliced summer squash and zucchini to the bowl, tossing gently to coat the vegetables in the marinade.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld. For best results, marinate for 1-2 hours.
After marinating, remove the bowl from the refrigerator and sprinkle the grated Parmesan cheese and chopped parsley over the squash. Toss gently to combine.
Serve immediately or chill for an additional 10-15 minutes before serving for a refreshing dish.