For me I dont believe he particularly stenciled in August 7th 85 as his d day.
He had talked about getting rid of all the family in the house before by burning it down. Which to me means that shelias visit with the boys to WHF was probably a regular bi monthly occurance. Opportunity having presented itself many times.
For me he got pissed off beyond comprehension this day and geared himself up for it. Possibly an argument with Shelia in the field, he blew up and just said " fuck it im doing it tonight "