I have long thought that the time to expect a major SHTF grid down or similar level event would be on an ordinary workday in early winter when schools are in session (including private schools and colleges that tend to have longer holiday breaks and different winter/spring breaks than public schools). That is how the terrorists or nation-state that is attacking would yield the maximum chaos and damage. On average families are not as dispersed on holidays and weekends. Tuesdays through Thursdays are the most dangerous days in my opinion given the frequency of people taking long weekends, plus business travelers are mostly all heading home on Fridays.
I say early winter because of the added impact of winter weather once modern amenities cease to function. Few people who live in northern climates even own any clothing and footwear designed for prolonged use in really cold/wet weather. What they own instead are clothes that look the part but are only geared for the typical short duration outside type usage.