The dates you have listed are due to when people normally get paid so they are more like to spend money.
I noticed it, however then I got to thinking and realized that I don't spend money on game purchases just after receiving pay. On average it takes about four to five days before I buy any thing. So this combined with the large amount of different payday dates and the fact that I missed two sales in the game I am playing makes me wonder is there like a golden date for games?
Personally, I would be very careful about sales as the short term spike is quite high, it can also be followed with a lower then average spending curve after the sale and can also get your users into a position where they just wait for sales. The flipside is that sales can help convert non-paying users to paying and therefore they become much more likely to spend money in the game at a later stage.
I noticed this when I played as a free member, or in the games I am not fully invested in, I will just wait holding on to my premium currency and spending it on a sale that would give me the most for my effort.
However I don't think this is as true for paying customers in most freemium games is it?
I spend my extra money on the first sale of the month, or event if there is no sale and time is running out; If I don't spent the money on the game I just end up spending it on something else.
Maybe if you have $50 or more that you plan on spending on the game at once, then maybe you would wait for the next big sale.