The plan adoption date is the official date when a retirement plan, such as a Solo 401k, is formally established by the business owner—typically marked by signing the plan documents. This date is important because it determines when the plan becomes active and which tax year it applies to. Traditionally, the plan had to be adopted by the end of the business’s tax year to make contributions for that year. However, due to recent law changes, many business owners can now adopt a Solo 401k plan after the tax year ends—up to their tax filing deadline (including extensions)—for the purpose of making employer contributions. It’s important to note that employee deferrals still require the plan to be adopted by December 31 of the tax year.