The temporary shutdown of a website is a means to express support for a cause by closing one's website for a limited time (typically a day) and displaying a campaign message instead - a form of one day banner activism.
A temporary shutdown can draw attention to an important cause, such as censorship of websites, by showing what it would be like without access to the site.
Considerations for implementation
When participating in a temporary shutdown campaign:
- Consider whether this may also impact functionalities beyond the frontend, such as APIs/endpoints etc.
- Display a clear message to site visitors what is going on and why, incl. a link to more information on the cause
- If possible, return HTTP 503 to avoid negative SEO impact , 
SOPA Blackout 2012
Global Climate Strike 2019
Website shutdowns were promoted as one form of digital participation in the Global Climate Strike of 2019-09-20.
Climate Strike Tools
- globalclimatestrike.net offers an embeddable banner (hack for self-hosting by Matthias Ott)
- Wordpress plugin wp-climatestrike
- Kirby plugin kirby3-climatestrike
Climate Strike IndieWeb Examples
- Sebastian Greger is going to shut down his site using his Kirby plugin
- @m_ott "My personal site just joined the global #ClimateStrike. It now shows a banner and will go completely green on Sept 20." 
2020-06-02: Appparently started by the Hollywood music industry(also), in response to the murder of George Floyd, #BlackOutTuesday is a call for “a day to disconnect from work and reconnect with our community" and “an urgent step of action to provoke accountability and change” often also with the hashtag #TheShowMustBePaused.
Many sites have blacked out their home pages, some with lists of black individuals who have lost their lives to police brutality and others. E.g.:
- ... add your site here
How to / web component:
- https://github.com/trentmwillis/i-stand is a web component you can insert into your home page or a site global template to display a similar message.
While not a full shutdown, CSS Naked Day—disabling the visual styles of websites for a day—made use of a similar approach, using temporary site-wide changes as a means for advocacy.