Actually, there have been a number of times this year when the rankings have been updated on Sunday evening, but today may have been the earliest ever (I saw it about 2 pm Pacific time today). I believe the rankings are done out of the Florida ATP office, so if all of the Challenger and Tour results are in there by Sunday afternoon then the new rankings are usually posted. I suspect one or two people come in to work on Sundays and can get the info to their webmaster.