On a calm day, what determines the active runway?
Almost every airport that handles frequent traffic will have a Preferential Runway Use Policy. That policy will cover things such as which runways to use for departures and arrivals when parallel runways exist, and the preferred direction even in calm winds.

It's based on the best noise abatement routes/gradients, taxi distances, terrain, etc.

So much so the preferred direction can be used in tail-wind up to a limit (8 knots component in certain places for example). Basically it's the usage that was envisioned when the airport was designed (which does take into account the historic wind data).




  Westerly Operations is the normal traffic pattern used at LAX during the daytime (6:30 am to
  midnight) throughout the year. Aircraft approach the airport from the east and depart the Airport
  to the west due to the prevailing westerly wind. This procedure routes louder departing aircraft
  to the west over the ocean, while arriving aircraft fly from the east to the west over the
  communities on the eastside of LAX, including the Cities of Los Angeles, and Inglewood, and
  the communities of Athens and Lennox.


Above (source) being the normal traffic pattern, it will remain so during calm winds.



Related:


In calm winds, which runway should I select at an untowered airport?
Why would a commercial flight be landing with a tailwind?