To a large mixing bowl, add olive oil, lemon juice, salt, pepper, and parsley, and whisk to combine.
Add the chopped dates, breaking them apart with your fingers. You can also use a whisk or fork as needed to break them up completely.
Add the celery, celery leaves, mandarin oranges, and pistachios (optional) and mix with two spoons until all the ingredients are evenly distributed. Taste and adjust as needed, adding more dates for sweetness, lemon juice for brightness, or salt to taste.