It was a day after Abraham's brith, a very hot day,
G-d  took the sun out of its shell ( the sun was in its maximum strength) just to give Abraham rest from receiving guests, 

but Abraham even with pain didn't give up to himself. 
So he waited outside his tent, to have guests, to be able to give something to someone even that day. 
So G-D sent him guests, 3 angels looking like 3 Arabs.
Such an impacting story were Abraham taught us about the importance of the act of giving every day of our life's.