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OR (Zyczymy) Pogodnego Festiwalu Jesiennego Or Dobrej Zabawy w Czasie Festiwalu Jesiennego
Explanation: Happy Fall Festival - in a banner, means that you wishing people good time.
In Polish you say "zyczymy" (plural form), or "zycze" (singular form).
Happy= szczesliwy, but you also can say - pogodny, radosny, dobry (czas), udany, wspanialy
Fall = jesien
Festival = Festiwal OR Karnawal (usually games, music, and fun)
So, you can say:
- Zyczymy Pogodnego Festiwalu (or Karnawalu) Jesiennego
- Zyczymy Radosnego Festiwalu (or Karnawalu) Jesiennego
- (Zyczymy) Dobrej Zabawy w Czasie Festiwalu Jesiennego (it means - have a great fun !)