> In the absence of calamity, fortuitous events, or brilliant new marketing strategies, sale counts are well-described by a Poisson process. That is, you can think of there being an underlying average number of sales per day, and each day will be a realization of a Poisson distribution with that average.

Can someone give a bit more justification for this? It seems like the average rate shouldn't be constant and is heavily dependent on time/date.

If not, is there another justification for why sales mean should equal sales variance?