The black suit with a white shirt and a black necktie is the traditional funeral outfit in many occidentally-influenced cultures. The colour black in the
context of mourning since Roman times, as Ovid described it. (1)
(1) The black of the nigrae vestes is the "colour of woe" for Ovid in Ibis 101-4, see Alexandra Croom's Roman Clothing and Fashion,
Amberley Publishing Stroud UK 2012. p. 21; also Harvey, J. Men in Black, Reaktion Books Ltd. 1995.
The other main occasion for wearing a black suit is an evening out in a nice environment, e.g. an elegant dinner, the opera etc. A bow tie that is not plain
back works best; the latter is the preserve of Black Tie. A subdued or creative neck tie, depending on the occasion, might be
Additionally, in some professional situation, e.g. some Court appearance in certain jurisdictions, a black suit is advisable.