The word "tithe" literally means "a tenth," and the practice of tithing to God involves giving a fixed 10 percent of one's income. Tithes are to be given on an ongoing basis as one receives income.
Offerings are a separate requirement by God, with two important differences from tithes: the amount and the frequency. Offerings were to be given on God’s holy days, which are His annual appointments throughout the year, in an amount determined by the worshiper.
God's law of tithing is revealed to us through His commands to ancient Israel. Because their economy revolved around the agricultural cycle, tithes were explained and revealed to them in those terms. As crops were harvested at various times throughout the spring, summer and fall of the year, a tenth of each harvest was set aside to be given to God (Leviticus 27:30). Likewise, as animals were born throughout the year, the tenth one to be born was set aside as tithe, "holy to the LORD" (Leviticus 27:32).
In other words, the tithe was set apart as the income was received, and the amount was determined explicitly by God. Therefore, in modern times, it is best to tithe as one receives income, setting aside one tenth of each paycheck and donating it regularly.
The amount of an offering is not commanded by God, but instead determined by the worshiper, "as he is able, according to the blessing of the LORD your God which He has given you" (Deuteronomy 16:17). This requires each person to examine how they have been blessed by God and decide for themselves how much they should give as an offering to show thanks to God for His blessing.
Unlike tithes, offerings are commanded to be given at appointed times as a component of worship on God's Holy Days throughout the year. God commanded that offerings be brought during each of His festival seasons, and that we "shall not appear before the LORD empty-handed" (Deuteronomy 16:16). Offerings are therefore required by God at these times.
Offerings can also be made spontaneously at any time as an act of worship when a person feels moved to do so, but this is separate from the commanded yearly offerings.
The United Church of God celebrates God's annual festivals throughout the year as Jesus Christ did, according to God's Law: "The feasts of the LORD, which you shall proclaim to be holy convocations, these are My feasts" (Leviticus 23:2). When we meet for services on these feast days, we collect an offering that is separate from the tithes that our members regularly contribute.
While we accept offerings at any time through the year, we do not collect offerings at our weekly Sabbath services since they are not commanded or expected at that time.
Since God is both the Creator of all things and the source of all physical blessings we have. He exercises the right to require a portion of it back through tithes and offerings. It is important that we understand how to properly "honor the LORD with your possessions, and with the firstfruits of all your increase" (Proverbs 3:9).