In the US most stores gear up for large increases in customers for the first of the month.... That is because in the US the first of the month is when most states hand out welfare checks & when social security & social security disability are paid out. For most people who are working in the US they generally get paid every other week (I've only met a few people who get paid once a month, and they generally work as contractors, vendors, or some such where their wages are tied to product that they sell).
If you are getting a lot of US sales, I wouldn;t depend on the first of the month, although I do not see much of a difference in sales from week to week- for the most part mine are steady.