The best time of day for family photos in Pennsylvania
The best light of the day lasts about seventy five minutes, and most families schedule their photos completely outside of it. Here is when to actually go, month by month.

The short answer
Start about seventy five minutes before sunset and shoot until the light goes. That is it. Everything below is the reasoning and the actual times.
Golden hour in Central PA, by month
Approximate times for the York, Lancaster, and Harrisburg area. Sunset shifts by a minute or two a day, so treat these as the shape of the season rather than a stopwatch.
Why evening light is better
At noon the sun sits almost directly overhead. That puts shadows in everyone's eye sockets and under their noses, and it makes people squint. There is no fixing that afterward. It is the single most common reason a family looks at their photos and feels that something is off without being able to say what.
In the last hour before sunset the sun drops low and its light travels through far more atmosphere before it reaches you. That does three things at once. It goes soft, so shadows have gentle edges instead of hard ones. It goes warm, which is flattering on every skin tone. And it becomes directional, which gives faces shape instead of flattening them.
The practical result is that in golden hour, everyone looks good. Including the people who will tell you they never photograph well.

What about overcast days?
Good news, and most families do not believe me until they see the gallery. An overcast sky is a giant softbox. The clouds diffuse the sun across the whole sky, which means soft even light with no harsh shadows and no squinting, and it lasts most of the afternoon rather than seventy five minutes.
I do not reschedule for clouds. I reschedule for genuinely unsafe conditions. Light rain with a family that is game for it makes some of the best photographs I take.
The exception: very young children
Golden hour in August lands at seven o'clock, which for a family with a toddler is straight through the witching hour. A tired two year old will produce a worse gallery than mediocre light ever could.
If bedtime is a real constraint, the answer is not to shoot at noon. It is to shoot in open shade in the late afternoon, or to book in October and November when the good light arrives at half past four. That is one of several reasons autumn books out first around here.
Sunrise, honestly
Sunrise light is just as beautiful as sunset light, and I will happily shoot it. In practice almost nobody books it, because getting a family of five dressed, fed, and somewhere scenic by six in the morning is a genuinely difficult thing to ask. If your family is up early anyway, it is worth considering. The parks are empty and the light is exceptional.
A planning rule of thumb
Take the sunset time for your month, subtract seventy five minutes, and that is when you should be standing at the location with everyone's shoes already on. Arriving at that time rather than leaving the house at that time is most of the difference between a relaxed session and a rushed one.
And build in fifteen minutes of slack. Someone always needs the bathroom.
Know when. Not sure where?
Six parks in York County worth your golden hour, with the light and timing for each.