I was reminded by an article in Beecraft of the cross over point.
It’s the point at which the death rate of winter bees is matched by the hatching of the new seasons worker bees, and it is the point at which the colony swings from dwindling to increasing in numbers.
Not that important then?
It is the critical point at which the over wintered colony pulls through well and truly, and prepares to take off in strength.
What could cause it to not succeed? Varroasis, nosema, acarine and starvation mainly. By this time bird and rodent predation are past being dangerous for most colonies.
What can be done to maximise the chances now? Make sure they have food to keep them supplied until the first flows. It is still Winter, and there are a good 4 to 6 weeks of dearth for the colonies to face yet.