The time of year that the birds really need you to put out food in your garden is during the winter months (roughly November-March). During these months, the natural food sources of fruit, berries, nuts and seeds have been depleted and there are few invertebrates active and available. Furthermore, the weather is cold meaning small birds have to use more energy to stay warm, and the days are shorter meaning there’s less time to find food to recoup lost energy.
That being said, it’s perfectly fine to feed them all year round if you want. During the summer months, adults have to feed themselves and their chicks. If there’s food in your garden, they will feed themselves on that, and that gives them more time and energy to find protein-rich foods for their chicks. It’s best to avoid fat/suet-based products during the spring and summer months, but do put out mealworms, peanuts and sunflower seeds.