The better days to hit businesses is Tues-Thurs. (for both direct snail mail and email)
The conventional wisdom is that on Mondays folks are busy catching up, and "getting into the grind" of the work week. On Fridays they are thinking about the weekend. However, like all things, a lot of depends on your package, offer, etc. My experience is that a good package and offer, no matter when they receive it, will be opened, responded to, etc.
The problem with direct mail (that is "snail mail" postal service) -- unless you are mailing (I'm assuming you are in the USA) first class and local, you will have no idea when it will be delivered. Even with first class local (which should only take one day) -- it's questionable.
Do your best to plan - but with snail mail, but I've experienced all kinds of absurd delays. Standard letter mail (US -in a #10 envelope seems to be more predictable)
If you are mailing local first class, I would suggest dropping on a Monday. In the perfect world it will arrive locally on Tues, Wed. or Thurs. If you are mailing regional or national... frankly that is too hard to predict..it's a gamble