Don’t Talk for God

I often read from postings on different discussion groups, Hebrew Roots, Messianic and Christian, of people telling others exactly what is right and wrong in God’s eyes.
Now, there are many things God tells us about Himself, but there are many things He doesn’t.
God gave us commandments, rulings, judgments and statutes (613, to be exact), but what about those things not mentioned? If God says to celebrate a certain festival, does that mean we cannot celebrate any other kind of festival? Too often I read people stating absolutely if it isn’t given by God, then whatever is man-made is wrong and against God.
My question is: “Who are we to decide what is God’s will?”

Comments welcomed (just be nice)