The "Deadline for Self Assessment registration to notify HMRC of Income/Capital Gains Tax" refers to the final date by which individuals in the United Kingdom must register themselves for Self Assessment with HM Revenue and Customs (HMRC) to declare their Income Tax or Capital Gains Tax liabilities. This deadline is significant for individuals who have not previously registered for Self Assessment but have income or capital gains that require them to report and pay taxes. By registering before the deadline, individuals ensure compliance with HMRC regulations and avoid potential penalties for failing to notify HMRC of their tax obligations. It's important to note that the specific deadline for Self Assessment registration may vary depending on individual circumstances, and it's advisable to consult the latest HMRC guidelines or official resources to determine the exact date applicable to your situation.