The temporary shutdown of a website is a means to express support for a cause by closing one's website for a day and displaying a campaign message instead - a form of one day banner activism.
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
Climat 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.
CSS Naked Day
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