mine starts sunday. this is roughly how i do it, sometimes i go a little quicker and introduce animals earlier.
1st week of April i turn the heating off for one hour per night. (i normally have heating 24/7).
Every week after i continue to cool by an extra 1-2 hours.
If they haven't gone off their food by now i stop feeding 1st week of May to ensure they have digested everything before it gets cold.
by sometime in june my heating is on 12/ off 12 hours. As soon as i get to the 12 hours on/off cycle, i introduce the breeders. Usually i try to introduce on 1st june.
This works for me, im sure there are many other ways.