Early Participation in Regulations Act of 2021 This bill directs agencies to publish advance notice of a proposed rulemaking at least 90 days before publishing a notice of proposed rulemaking for a major rule that the Office of Information and Regulatory Affairs (OIRA) determines is likely to impose (1) an annual effect on the economy of $100 million or more; (2) a major increase in costs or prices for consumers, individual industries, government agencies, or geographic regions; or (3) significant adverse effects on competition, employment, investment, productivity, innovation, health, safety, the environment, or the ability of U.S. enterprises to compete with foreign-based enterprises. The advance notice must include, among other information, a written a description of the rule and the legal authority under which it is proposed; and solicit and provide a period of at least 30 days for submission of written data, views, and argument from interested persons. Any difference between such advance notice and the notice of proposed rulemaking may not be considered arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with the Administrative Procedure Act. Advance notice is not required if the proposing agency is not required to publish notice of proposed rulemaking or OIRA finds that such advance notice is (1) not in the public interest, (2) duplicative of another statutory requirement, (3) not practicable due to a required deadline, or (4) for a rule that is routine or periodic in nature. Such a determination made by OIRA is not subject to judicial review.