Global Contribution Day is an event that encourages participants to contribute code or test issues for Magento open source projects. Participants are instructed to choose a project from the list of focus projects, select an issue to fix or test, follow the contribution process of forking a repository and submitting a pull request, and ensure all tests pass. The document provides links to Magento coding standards and testing resources and emphasizes testing issues by verifying them and applying appropriate labels to document the results.