Paolo A. answered 04/01/19
Experienced Latin teacher, Ivy League PhD
'Prossimo' literally means 'the closest', hence 'next' in expressions of time, as in 'domenica prossima', 'next Sunday'. In common speech, however, Italians have lost the original sense of 'prossimo' as 'next' ('the closest'). 'Questo sabato' = 'this coming Saturday' should always be used as completely synonymous with 'Sabato prossimo', but many Italians prefer to use 'prossimo' to mean something like 'the Saturday closest to this coming one', hence the ambiguity.