When God gave Moses the 1st 5 books of The Bible, A mandatory sacrifice was written into the law. Everyone was required to give to this sacrifice during the year. It was for a sin Offering. Although this offering was mandatory God wanted people to give out of the abundance of their heart. So what God established was a free will offering. This was an offering that you could give at any time of the year. Just when you were moved by God and you felt that it was necessary to express Thanksgiving. A free will offering is not given because you have to, it was given because you wanted to. This is the only offering where the priest could eat from. In psalms 100 and says to enter his gates with Thanksgiving into His courts with praise. This is how you to give a free will offering by wanting to give to God because he had blessed and kept you all year. And this is how God wants us to go to Him. This is our free will offering, A thankful heart.