The quick answer is...not as important as you might think. The long answer is that there are a lot of more important factors that determine how long you will wait in line. The time of year, the time of day and the attractions you choose to experience will have a much greater impact on how much time you spend in line.
|Time of Day
|Extra Magic Hour Morning
|Day of the Week
So what does this tell us? According to our data, the attractions you visit and the time of day at which you visit them will have the greatest influence on how long you wait. On the other hand, the day of week and the weather will play only a small role. Statistically speaking, the time of day is 5 times more important than the day of the week.
Still, there can sometimes be an advantage to selecting one park over another on a particular day. For more on that, check out our Walt Disney World Park Recommendations page. (It's less relevant at Universal Orlando and Disneyland, where there are only two parks.)