This is a fabulous time of year as it is the time when your deciduous trees start to push leaves and all trees start to increase their productivity after winter dormancy.
Prior to the full onset of Spring repotting should have taken place to root prune the trees and check the composition of the soil plus look for soil pests and replace the soil as necessary and add any additional nutrients. The most common repotting months for bonsai are around February to March but depending on your trees and their development this may vary for you.
Therefore subject to when you repotted you can commence feeding in spring as long as this is around 4-6 week after any trees have been repotted. Many trees have started to flush already and some are already in full leaf but be careful in moving trees out from any form or protection as the evenings and nights still cool down and if the leaves are new they are vulnerable to cold conditions which can set them back.
As the trees buds start to swell this is the time to increase your watering regimes to aid with the flower or leaf development whichever comes first. For example early flowering trees that flower before coming into leaf include Prunus Mume, and Forsythia while Malus tend to flower after coming into lead.
The seasonal timing of natural events (Phenology) in spring revolves around flushing and bud burst including flowering for some species, which is then followed by fruiting and autumn leaf fall. So subject to the environmental factors in which you keep your bonsai trees, which affect the timings of these events.
For example if you keep your bonsai trees in a green house over winter they will tend to come into leaf earlier than someone who keeps their trees outside all winter. However, as this winter has been so mild this may not have induced a deep dormancy for the trees so they may come into leaf sooner.
Here are a few images of trees that are just coming into leaf or displaying well as present for your enjoyment.