Instructions for the second tithe are found in this passage. Included here is the command to "truly [fully] tithe all the increase of your grain that the field produces year by year" (verse 22). This is the tithe we are to save for ourselves for use during God's Feasts throughout the year, not the same tithe that was given to the Levites.
Note here that a full tithe is to be set aside for this purpose only. Over the years some have invested their second tithe in projects that have failed; this is a wrong use of this money. It is not to be set aside as venture capital, but as money to be used to enjoy God's festivals.
Some have erroneously thought that if they saved just enough to attend the Feast, they were fulfilling God's requirement. Again, this is not correct, for God wants us to save a full tithe to "spend on whatever your heart desires . . . before the LORD your God, and you shall rejoice, you and your household" (verse 26).
Also, spending second tithe for any other purpose than for keeping the Feasts is wrong. It actually harms our relationship with our Creator. We destroy character through disobedience and fail to learn the important lessons contained in the saving and use of this second tithe.