Either form would be proper depending upon what you wish to convey.
The plural form of Wednesday is Wednesdays
The noun Wednesdays is the plural form for the singular noun Wednesday.
In 2010 there are 52 Wednesdays.
There were 52 Wednesday's in the year 2009.
If you mean to say "Wednesday 37 times", type "thirty-seven Wednesdays". If you mean to say "The event that belongs to Wednesday", type "Wednesday's event". If you mean to say "You'll see it with your own eyes if you're here on any Wednesday", type "Wednesday's the day!"
Orange Wednesdays offers cheaper cinema tickets, trailers and regular discounts on Wednesdays. So you could say that Orange Wednesday promotes the film industry.
"He plays soccer on Wednesdays"; or you could say, "He plays soccer on Wednesday afternoons" (or mornings: which ever is applicable).
There were exactly 52 Wednesdays in the year 2010.
There were 53 Wednesdays in 2008. 2008 was a leap year that began on Tuesday and ended on Wednesday, so there were 53 Tuesdays and Wednesdays, and 52 each of every other day of the week. == ==
There were only four Wednesdays in October 2009.
Exactly 14% of all 12ths of January are Wednesdays.
All months have a third Wednesday. May is the only month in 2017 that the third day of the month is a Wednesday.