You'll need to change the attribute to:
OnBeforeEventRender="DayPilotMonth1_BeforeEventRender"
or add "Handles" to the the method signature:
Protected Sub DayPilotMonth1_BeforeEventRender(ByVal sender As Object, ByVal e As BeforeEventRenderEventArgs) Handles DayPilotMonth1.BeforeEventRender
'...