Yes, you are absolutely allowed to leave and reenter Disneyland Park or Disney California Adventure on the same day, provided you have a valid admission ticket. When you leave the park, you do not need a hand stamp anymore; your physical ticket or the digital barcode in the Disneyland app is scanned upon reentry. This "in-and-out" privilege is a popular strategy for families who want to return to their hotel for a midday nap, a swim, or a more affordable meal before returning for the evening fireworks. If you have a Park Hopper ticket, you can even leave one park and enter the other (typically after 1:00 PM, though this time is subject to change). It is important to keep your ticket or phone handy, as you will have to pass through the security screening and the turnstiles again. Also, be aware that leaving the park does not guarantee reentry if the park reaches its maximum capacity—which is rare but can happen on extremely busy holidays like Christmas or New Year's Eve. Your parking pass is also valid for the entire day, so you can leave the parking lot and return without paying again.