There are different deadlines for doing this, depending on whether you have a site or an app, and when it was published.
New websites (published on or after 23 September 2018) – by 23 September 2019
Older websites (published before 23 September 2018) – by 23 September 2020
Mobile apps – by 23 June 2021
Accessibility means making sure that your site, tool, or app can be used by people with the widest range of capabilities.