originally a calling away, an interruption, a distraction, is used in Britain, though not in U.S., as a stylish synonym for vocation or calling, which it is properly in antithesis. This has destroyed a useful differentation; but vocation has retained the idea of ‘calling’; a man is fortunate if his avocation, in the sense of his occupation, is also his vocation.

H.W. Fowler
A Dictionary of Modern English Usage
2nd Edition