This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Occasionally, attractions at Disneyland Resort will be scheduled for refurbishment. The attraction will close for a period of time to allow maintenance beyond that which can be handled during routine daily rotations. Some scheduled maintenance also allows for the addition of new elements or for attractions to be switched over for new seasons (think the addition of Nightmare Before Christmas characters for the holiday seasons). You can find a schedule of refurbishments on the Disneyland app or website, though you should keep in mind that these schedules are subject to change.