in my area the council is particularly stupid regarding this form
you return the form stating who was residing at the address on one day in october say the 16th
i get the form in august
a reminder in september
a jobsworth knocking on the door at the end of september
i complete the form online on the day following the date in question
if they want to know earlier set the date earlier - simples
wait for the next national census... try sending that back a month early