As for me, I started out thinking that Shall v. Will was like any other grammar mess with a simple rule to solve the problem. However, this is no ordinary mess, and I have ended in doubts as to what I thought was my deep and abiding love of intricate grammatical nuances.
If your grammar love roots grow deeper than mine, perhaps you can make it through the Wikipedia article on Shall v. Will. However, after looking briefly at all of the subheadings in that section, I decided that I would rather prune my grapevines. (Roses would have brought me back in to blog, the nasty thorned things.)
A historically infamous shall usage was when General Douglas MacArthur said, "I came out of Bataan, and I shall return," meaning that he wanted to go back and fight. But after the Bataan death march, it turned out to have been prophetic in a macabre way, since he had returned, but over 1,000 Americans and 5,000 Philipinos did not return. Politicians asked him to change his speech to "We shall return," and he didn't do it.
The basic rule in the past has been: Shall means an obligation and will means desire. So the English used to joke that a misinformed person might cry out: "I will drown and no one shall save me," and the declaration would be interpreted as a resolution to commit suicide.
There is your mell of a hess for the day. Lucky for us, grammar is not generally a matter of life and death.