There are different fertilizing schedules for young citrus trees and mature or fruit-bearing citrus trees. However, the idea is to fertilize them during the dormancy period so they can have the energy and nutrients they need for the spring season.
You should start fertilizing young citrus trees from January to June or July. If your citrus trees are in their second year, you can fertilize them from January until September. As the trees start to mature and bear fruits, you can now lessen the application times and feed them in January, mid-April, and September.