LOS ANGELES — For the first time, tickets to Walt Disney World in Florida and Disneyland in California will cost more during holidays and some weekends — up to 20 percent more — than during slower periods, as the bursting-at-the-seams parks seek to spread out demand.

The Walt Disney Company unveiled its long-expected move to demand-based pricing in a blog post on Saturday. The seasonal changes, which take effect on Sunday, affect the two theme park resorts differently and apply only to single-day tickets and not the multiday packages that the majority of vacationing families buy, particularly at Disney World.

At Disneyland, located in Anaheim, Calif., which attracts roughly 17 million visitors annually, single-day tickets now cost $99. Starting on Sunday, the park will charge three different prices based on the calendar. “Value” tickets, for Mondays through Thursdays during weeks when most schools are in session, will drop to $95. “Regular” tickets (most weekends and many summertime weeks) will climb to $105. “Peak” tickets (most of December, spring break weeks, July weekends) will cost $119.

At Disney World in Orlando, Fla., which includes four major theme parks, the price changes are more complex, because they vary by park. At the most popular Disney World park, the Magic Kingdom, which handles nearly 20 million visitors annually, single-day prices will remain at the current level, $105, for value periods. Prices will rise to $110 for regular periods, and to $124 for peak.