He waited seven days, the time set by Samuel; but Samuel did not come to Gilgal, and Saul's men began to scatter. So he said, "Bring me the burnt offering and the fellowship offerings. " And Saul offered up the burnt offering. (1 Samuel 13:8-9 NIV)
To understand what Saul is doing wrong here, you will need another scripture. In 1 Samuel 10:8 Samuel says to Saul "Go down ahead of me to Gilgal. I will surely come down to you to sacrifice burnt offerings and fellowship offerings, but you must wait seven days until I come to you and tell you what you are to do."
What did Samuel, the man of God, tell Saul to do? He told him to wait, didn't he? He asked Saul to wait seven days. Samuel had told Saul that he would be there and that he would do the burnt offering at the end of seven days. Saul was supposed to wait it out. Why didn't he?
Because Saul took things into his own hands. He decided, on his own, that Samuel was taking to long. Saul saw his men afraid and running away, and he began to get discouraged. He took things into his own hands, and believing that the burnt offering would bring God's favor, he sacrificed there before the Lord, and before Samuel could arrive.
Have you ever taken things into your own hands? Do you sometimes fail to wait on God, before you start to panic? Do you get restless if you have to wait?
I can tell you now that, as you read the rest of this story, Saul is going to lose far more than he gained, by not waiting upon the Lord.
