I've looked all over but I can't seem to find out when you're supposed
to do this ?

We've a large apple planted in 1933 that's all over the place. The last
owners seemed to have an aversion to going into the garden at all (we've
cleared 6 skip loads so far).
I've been told to cut it all the way back (it Y's off at about 5
feet and I was told to go about 4 foot along each branch and cut down
from there). Is this the right time of year to do this, I really,
really don't want to stuff this up.

Very many thanks
--
Steve