When they decide to... not meaning to be mean or anything, but different stores have different hours. I would guess 9am or 10am depending on city / location. There is nothing I have seen where Apple is opening early.
My store (closest but still 140 miles way) is opening early at 8:30 am on Friday. So I was wrong ... above. My apologies. Check your store, seems the hours on Google show updated / different hours for Friday.