The difficulty in translating the phrase 'have a nice weekend' into Latin comes in the different approach to the calendar taken by the Romans compared to ourselves.
Although they also had the seven days as we do, the Romans did not count time in weeks, and so would not consider Saturday and Sundays to be the end of a set period of time.
The closest the Romans had to a word for 'week' is hebdomadis which means 'seven days'. If a Roman understood you meant Saturday and Sunday as the end of the latest series of seven days, you could say interdum modo elabitur hebdomadis finem, which very roughly means 'have a nice seven day's ending.'
You could of course wish your Roman friend a nice Saturday or Sunday, which would translate as habere boni Saturni or habere boni Solis respectively.
Roman Timekeeping Doesn't Translate Very Well
When it comes to the Romans, we must remember not only their language was structured differently, but they also looked at time in a different way, partly due to the technology they had at their disposal.
Without mechanical timekeeping, the Romans relied on the daylight to tell their time across the various latitudes of their empire. Hours were not only of a different length in their various territories, but also of varying lengths with a single day!
Confused? This instructional YouTube video will help explain: