We put article when it is subject, when it is object it is not necessary if you talk in general sense.
- Camino en verano. (All Summers, you walk)
- Camine durante el verano. (You walked in the Summer you are talking about)
- Tenemos trabajos de verano. (they are not from a specific Summer)
- ¿Recuerdas tu trabajo del verano pasado? (A concrete Summer, so you need article)
In general articles are very similar to English when you are using a noum as object, but we always use an article when the noum is used as subject.