I’m rescheduling a rock climbing appointment. I say:
The appointment has to be for/in September.
"For September" sounds as if it will be for the whole month. Compare "They booked a holiday cottage for September" or, if they were not so rich, "for the second week in September" or "for the last weekend in September". " If it just for a day or weekend, use "in September"
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"For September" sounds as if it will be for the whole month.
Compare "They booked a holiday cottage for September" or, if they were not so rich, "for the second week in September" or "for the last weekend in September".
Similar to "He worked in Portugal for three years."
If it just for a day or weekend, use "in September"