As trees sense spring is starting (by a combination of longer daylight, increased temperature and stronger sun rays (caused by elevation of sun in the sky)), they send signals to buds to begin to swell.
The buds will enlarge and then leaf burst will occur.
Please note this is a chemically triggered event, it is not an absolute indicator of the health of a tree. The acid test is whether the tree supports the new leaves.
Just to confuse things late frosts can damage new leaves.
Aftercare of newly planted trees and stressed trees, should commence as the buds swell to support this process and continue through spring and summer. It is recommended to assist newly planted (and transplanted) trees whilst establishing for upto 5 years. Established is when the tree can support itself. During later extreme dry and hot weather, the trees will always thank you for additional help.