I've seen all sorts of combinations for cheaper flights over the years on various airlines including
needing a Saturday night stay - the most common. Some specify the Saturday night must be at the destination not in the air as Saturday turns into Sunday
Needing a Sunday Night Stay
Outbound on a Tuesday / Wednesday / Thursday and return on a Monday / Tuesday / Wednesday
Requiring a specific length of stay at the destination of a specific number of nights but not always a Saturday night