Damsons, plums, apple and pears

We have all 4 growing in the orchard, 2 of each and I've never got the picking right 

The pear tree is about 40 years old and has only 4 pears this year; one more than last year, although in previous years it was laden.  The other 3 we planted about 8 years ago, plenty of apples, a good couple of branches of plums (not Victoria) on one tree only and as usual very few damsons again on 1 tree only.

So when is the best time to pick

Grace