Well I have never worked at the park so Im assuming they look at previous years attendance and make a deduction of how many people they are expecting. But Im sure attendance is sometimes unpredictable. If weather doesn't look too cooperative then attendance will be low. Usually the park is slow when it first opens then highly picks up during the summer months when everyone is off of school and adults/parents are taking vacations. Also if the weather looks like its going to be uncooperative and maybe violent then they either decide not to open the park for the day or close the park early. I would call the park for that. Theres an option for weather closings on the automated phone service they have. Other than that the park is open for the whole day. As far as how they decide what hours the park normally closes and opens I do not know. More kids are around in the summer so they are open till late. Some days there will be tons of people and they will have only 1 car running while some days I've gone its been slow and they have 2 cars running. Its all about predictability. But like I said, I have never worked for the park, I'm just a fanboy so you would be better off getting advice from and actual employee.
Yeah, it's basically deals with how many people have come to the park on a given day, and them knowing how many people came before, so they choose what rides to staff, and what rides not to staff - such as low staffing days. With the low staffing days, they also worry about capacity rides. If a ride has a low capacity, that ride might not be staffed.
For the early season, a reason why they now open later than they did before is because they seen the trends of less people at the beginning, and thus they aren't opening as early. And some of the trends are because of the weather, but to me it also has to do with the new additions. If the park is getting something new, and the new ride isn't open yet, do you really want to waste your trip to the park if you are a 1 or 2 times a year type of person? Also, earlier on the hours aren't so good, so people factor that in too. People want longer and longer hours better than shorter hours. They feel like they are going more for their buck if the park is open 10-10 pm compared to 10 -6 pm.
The park looks at all these numbers about what everyone does at the park. I wouldn't be surprised if they have counters in the bathrooms. Even when you don't think you are being counted (such as no turnstyle), they still have counters - such as the exit. They also have separate counters for the waterpark as a whole - how many people have entered the whole day, how many people have left, and how many people are in the waterpark at a current time. Do they have a counter per slide / lazy river? I don't know? Maybe they do? Maybe, I should look for them..
"I've been staring at the world, waiting. All the trouble and all the pain we're facing. Too much light to be livin' in the dark. Why waste time? We only got one life. Together we can be the CHANGE. So go and let your heart burn bright"
Parks operating hours are planned based on the attendance projection that will average out to a certain amount of rides per guest. I don't know exactly what SFGAm's is, but most park executives are happy somewhere in the 7-10 rides per guest range (which can be calculated very easily by diving the total amount of rides given that day by the parks attendance).
In regards to SF you also have to keep staffing in mind, SFGAm's is rarely ever open past 10 which keeps the shifts consistent and makes running the park a lot easier. During Fright Fest in 06 I remember on Sunday's it was only open until 8 that way the employees only needed the one 30 minute break instead of two which made staffing the park a lot easier.
On the other side of the coin, Disney will keep the park open as long as it takes to get that target rides per guest, Magic Kingdom during Christmas week opens at 7AM every morning for resort guests (8 for day guests) and some nights stays open until 3AM for resort guests. It is pretty awkward being at the park when it opens and it's still dark out. Epcot's operating schedule is a lot more consistent, 9-9 every day of the year except Christmas week since it only really has 3 major rides that get long lines. These kind of hours are lot more feasible for a park like Disney or Cedar Point though because of all the people living in company housing near the parks that they can scheduled at any time for however many hours. Universal normally closes at 6 to push people into City Walk at dinner time, though obviosly not during summer or holiday times.
In regards to how they actually project the attendance I am not as familiar with that. However it's pretty easy to get the general idea based on the time of year, weather, day of week, promotions, and also depends on group sales/presold tickets. In a book I have there are some examples of how they calculate the projected attendance when doing feasibility studies for a new park though.
Predicting attendance is all about patterns. The longer a park is open, the easier it is to budget your park for an entire year.
For SFGAm, after being open for 35 years I am sure that they have guest patterns pretty down pat. You base your operating hours/staffing based on those numbers. You also take in account when the school year ends/begins, any major events that are happening (super bowl, 4th of July, new ride, etc) all months before the season starts.
Top 5 wood-5-Goliath 4-Ravine Flyer II 3-Phoenix 2-Voyage 1-El Toro Top 5 Steel- 5-Velocicoaster 4- Maverick 3- Fury 325 2-Steel Vengeance 1-X2 Coaster Count: 444
Not only for park hours, but it also works with park operating days. In 2009, the park opened April 3rd with only 40 cars int he parking lot. In 2010, they opened April 24th and still didn't get a big enough crowd to make the day profitable. So that's why they opened the park this year on May 7th with mass-advertising to make sure they had a huge opening day crowd.