Sentence:  I had <WSD>occasion</WSD> […] to make a somewhat long business trip to Chicago, and on my return […] I found Farrar awaiting me in the railway station. He smiled his wonted fraction by way of greeting, […], and finally leading me to his buggy, turned and drove out of town. I was completely mystified at such an unusual proceeding. 
Meaning of the word marked with <WSD> tags:  Need; requirement, necessity. (from 16th c.)