If they need it, it should be done in the spring. I assume that you mean in situ camellia, rather than container grown.

Start your fertilizing program after the first set of new leaves appear in the spring. Here in So. Cal that is usually in late March to early Spring. Use a low dose nitrogen fertilizer such as fish emulsion or cotton seed meal. Once the buds form stop the nitrogen and apply some sort of phosphorus like bone meal or a liquid to help with the development and opening of the buds in the Fall and Winter. I like to add iron, it helps with the red and pink flowers by deepening the color. I use a phosphorus fertilizer all through the blooming season. Camellias are dormat when they are blooming and the root system is growing, the phosphoruos helps with the root system.

I fertilize all summer with free used coffee grounds from Starbucks