For Greek, it seems to be Sunday. Cloudy Sunday (Συννεφιασμένη Κυριακή), the #1 sad song for the postwar generation. I want it to be a Sunday (on the day I die) (Θέλω να είναι Κυριακή). Hammer and Anvil (Σφυρί κι αμόνι) compares the singer’s life to Good Friday Mass; but it also asks for a nightingale to console his Sunday.
As to why: I offered in that comment thread:
My guess it, the end of the weekend is when you have more free time contemplate things.
As for Sundays and why they’re good songwriting material, I think you’re right, free time is certainly an element. I’d also add the fact that the next day is a working day (== sad) and that certain things in Greece mostly take place on a Sunday (football matches, weddings, elections etc.)