Hanuman was worried he would not get any mangoes. He knew the sun would show up tomorrow at dawn, just like any other day. He didn't want Peanut to catch him lying.
So that night, when everyone was fast asleep, Hanuman silently went to the mountain.
Hanuman went to the foot of the mountain then pushed the mountain slowly, so he would not wake up the entire valley.
The mountain moved slowly but steadily to the spot between Hanuman's tree and the sun at dawn.