There are two different kinds of early entry: Extra Magic Hour (EMH) and Magic Morning (MM). EHM is only available to guests staying at one of the three on-site hotels (
Disneyland Hotel, Paradise Pier Hotel, and the Grand Californian). As an on-site guest, you have access to EMH every morning of your stay (if offered). MM is available to guests with certain three-day or longer pre-purchased tickets. Each eligible ticket includes only one Magic Morning.
Normally, DL has EMH
and MM on Tuesday, Thursday, and Saturday.
Normally, DCA has EMH on Sunday, Monday, Wednesday, and Friday. DCA does not participate in MM.
Typically, DL and DCA do not have early entry on the same day, but it does happen, usually during the busiest times of the year. For example, during the peak Christmas period, DL and DCA both have early entry (DCA has EMH and DL has EMH & MM) on a daily basis. However, outside of those periods, the parks usually follow a pretty set schedule. On a similar note, the days can change if there is some special event occurring (e.g., marathons) or be cancelled (e.g., there is MM or EMH on the Saturday after the 24-hour day).
During MM & EMH in DL, only select attractions within Tomorrowland and Fantasyland are open. During EMH in DCA, select attractions throughout the park open (including RSR and TSMM).
More information is available at:
https://disneyland.disney.go.com/guest-services/early-admission-options/