It gets light enough to run record return runs by seven or so, so at six, when the pits open for the return run(ners) it'll be in the early stages of getting light.
As for why the pits open for hoi polloi on Sunday, not Saturday, contact the SCTA/BNI and chat with them. The course, pits, and other setup chores need to be complete before the gates are opened, so to speak -- you might be able to convince them to start the work a day earlier. You might also convince snowballs not to melt in August in Wendover.