This talks of one tenth set aside every year. Why is this given? Is this to help in the celebration of the Jewish holidays?

Shalom, Thank you for your question. The Torah in several places requires separating tithes and portions from agricultural crops grown in the land of Israel. Some of these gifts are designated for the Cohens, some for the Levis, some for the poor, and some are for the owner of the crop themselves to eat in purity in Jerusalem. The verses you refer to (Devarim 14, 22-26) talk about the second tithe. This is separated from produce of the first, second, forth and fifth years of the seven year sabbatical cycle. As the verses explain, the owner separates the tithe (one tenth of the produce) and takes it to Jerusalem to eat it there in purity. If this is difficult (because the produce will not stay fresh, or it is too far to travel etc) then the tithe may be transferred on to its monetary value. The money is then used to buy foodstuffs in Jerusalem. You are correct that perhaps one reason G-d commanded us to keep this law is in order to help celebrate the festivals in a more joyous way. There are other explanations also, as well as spiritual meanings to this command. The Ramban explains that the purpose of the command is to teach one true fear of heaven, which is gained by having to eat the food in purity in front of the Temple, as the do the Cohens with their holy food. Others explain that the purpose of the command is to connect every Jew to the Temple and its service, and not just the Cohens. It also comes as a way to strengthen the centrality of Jerusalem and the Temple. Today, unfortunately, we are impure, and unable to eat this tithe in purity. Therefore, we limit the fulfillment of this command to its minimal definitions – and await the time when we will be able to completely enjoy the command in all its fullness. Blessings.