Questions & Answers about В субботу мы собираемся устроить пикник у озера.
Because with days of the week, Russian usually uses в + accusative to mean on a particular day.
- в субботу = on Saturday
- в понедельник = on Monday
- в пятницу = on Friday
Here, суббота is a feminine noun, and its accusative singular form is субботу.
So:
- В субботу мы собираемся... = On Saturday, we’re planning...
в субботе would not work here for a time expression.