Forty years later, I think we have mostly forgotten just how complicated and dangerous it was to put three men on top of a 6.4 million pound rocket and ignite it. The launch team was confident after several successful Apollo launches, but everyone knew that there was a risk of something going disastrously wrong and the crew dying a fiery death. Instead of celebrating the first men to land on the moon in five more days we could have been watching a solemn memorial service.
In the meantime, the launch team continued the many tasks needed to bring the rocket to launch ready status. Starting on the evening of the 15th, the fueling task was begun(described in more detail here). The first stage of the rocket burned a kerosene/liquid oxygen mix and the millions of pounds of kerosene was loaded first. Later in the night/early morning of the 16th, they would begin filling the cryogenic tanks with liquid oxygen and hydrogen and maintain those at low pressure until just before liftoff.