With door knocking the best times is always when someone is at home, if you door knock in the day time you will only get the ladies who look after the home, the home workers and the night shift workers, who won't be amused at being woken up.
The best times I have found is to be between 16.00hrs and 19.00hrs.
The reason for these hours are as follows, at 16.00 the day ladys have picked up their children from school and are at home, the night workers have woken up re-freshed for the day to come and the day workers are coming home.
I would not recommend knocking after 19.00 beause firstly the parents are either eating or enjoying their time with their children before bedtime and secondly the dark nights can be a security issue.
Also when you knock the door always step back from the door by at least 2 paces, a lot of people are less treatened by this than if you are in their face when they open the door.
If you are going to employ someone to door knock then train them to always smile when the door opens and always kept eye contact, the reasons for this are if you smile at someone they will always drop their guard and smile back therefore breaking the ice, keeping eye contact shows that you are trustworthy.
This advice is free of charge but you can make a donation to NSPCC if you like.