What is the context for the use of 'feira' when talking about the days of the week (e.g., segunda-feira)?
It comes from latim meaning "rest". At first, it was to designate just the days on the week of Christ's death and they should rest all the week long. Segunda-feira: second resting day after Domingo. Further information: http://mundoestranho.abril.com.br/materia/por-que-os-dias-da-semana-tem-feira-no-nome