Vote for your three favorite entries:
@Marynator #1:
The Nativity of the Blessed Virgin Mary (September 8th)
@Marynator #2:
National Yoga Month
@Avro Arrow #1:
Day of the Programmer (September 13th)
@Avro Arrow #2
Star Trek Day (September 8th)
@Kai "the spy" #1
International Literacy Day (September 8th)
@Kai "the spy" #2
German Butterbrot Day (September 25th)
@auntiehill
Talk like a Pirate Day (September 19th)
@rhubarbodendron #1
German Day of the Zebra-Crossing (September 1st)
@rhubarbodendron #2
Japanese Chrysanthemum Festival (September 9th)
@Dimesdan #1
Oktoberfest (starting mid- to late September)
@Dimesdan #2
All-Ireland Hurling Final (traditionally early September)
@Marynator #1:
The Nativity of the Blessed Virgin Mary (September 8th)
@Marynator #2:
National Yoga Month
@Avro Arrow #1:
Day of the Programmer (September 13th)
@Avro Arrow #2
Star Trek Day (September 8th)
@Kai "the spy" #1
International Literacy Day (September 8th)
@Kai "the spy" #2
German Butterbrot Day (September 25th)
@auntiehill
Talk like a Pirate Day (September 19th)
@rhubarbodendron #1
German Day of the Zebra-Crossing (September 1st)
@rhubarbodendron #2
Japanese Chrysanthemum Festival (September 9th)
@Dimesdan #1
Oktoberfest (starting mid- to late September)
@Dimesdan #2
All-Ireland Hurling Final (traditionally early September)
For me it's mostly market day, laundry day and tomato-preserving day, though. And tidying up day, freezer cleaning day, quilt finishing day and a gazillion more. Mostly, it's weekends-are-always-too-short day *sigh*

(btw, do you happen to know someone who might be interested in PR first editions? Read and in no good condition, I'm afraid)