How about this?
if day in st:
print(f"In {year}, Easter will fall on {'March' if month == 3 else 'April'} {str(day)}st")
elif day in nd:
print(f"In {year}, Easter will fall on {'March' if month == 3 else 'April'} {str(day)}nd")
elif day in rd:
print(f"In {year}, Easter will fall on {'March' if month == 3 else 'April'} {str(day)}rd")
else:
print(f"In {year}, Easter will fall on {'March' if month == 3 else 'April'} {str(day)}th")