Sentence:  Ensign Vesole decided that he would not tarry until he heard the whispering of the bombs , and when night began to <WSD>fall</WSD> , he put Seaman 2 / c Donald L. Norton and Seaman 1 / c William A. Rochford on the guns and told them to start shooting the moment they saw an enemy silhouette . 
Meaning of the word marked with <WSD> tags:  occur at a specified time or place