"We will watch a film on Saturday."
Translation:Ми будемо дивитися фільм у суботу.
ся/сь are essentially the same thing, reflexive suffix. After consonant only "ся" is used (вмиваєшся, not вмиваєшсь - that's just impossible to pronounce). After a vowel any can be used: дивимось - дивимося, збирались - збиралися. сь is just like shortened form.
However, there are people who argue that both variants are valid in Ukranian/are interchangeable (after a vowel):
- some people say that сь can be used only colloquially and not in any official writing
- there are people who are sure that сь is a Russian suffix and there's no place for it in Ukrainian. However, that opinion appeared only sometime in 2000's. Most likely as a consequence of "overcorrecting" in attempt to "clean" Ukrainian from everything Russian (another example of that process is thought that "спасибі" is a calque from Russian which is plain stupid)
- and finally there are people who advise to use сь before vowel and ся before consonant, but examples from literature show that such rule never existed