Here are the suitable words to complete the gaps:

1. celebrate
2. traditional
3. spring
4. Therefore
5. kept

So the completed text would read:

Tet is the biggest festival in Viet Nam. To **celebrate** Tet, Vietnamese people make many tasty **traditional** foods. The most important food includes Chung cakes, sausages, boiled chicken, **spring** rolls, and sticky rice. Chung cake is made of sticky rice, pork, green beans, and other spices, wrapped in green leaves; **therefore**, this cake needs a lot of preparation. This cake can be **kept** for a long time, even though the weather is often humid during Tet.