After his vacation, he spent a whole day reacquainting himself with the new office procedures.

reacquainting 是动词 reacquaint 的现在分词形式。reacquaint 由前缀 re-（表示"再次"）和动词 acquaint（使熟悉、使了解）组成，意思是"重新熟悉、重新了解"。 在句中，reacquainting himself with...表示"让自己重新熟悉..."的动作。