With respect to your federal income taxes, there is no status of being "retired," only the status of having separated from service with a particular employer. The things I can think of that could potentially be affected:
- It might affect your required beginning date for RMDs from a qualified retirement plan provided by this employer. If you separate from service on December 31, 2023, the still-working exception with respect to a 2023 RMD would no longer apply (if that's relevant to your circumstances).
- For the purpose of making a deductible traditional IRA contribution for 2024, the separation date might affect whether or not you are covered by a workplace retirement plan for 2024 (assuming that the employer provides such a plan). This will also depend on whether the plan uses a calendar year or a fiscal year.
- For the purpose of making any IRA contribution for 2024, it could affect whether or not you have supporting compensation in 2024.
- Working in 2024 will increase your 2024 income, even if only slightly.
There might be other considerations with respect to your state income taxes.