Retirement tax questions

Okay, great, thank you. More questions:

What if I retire on Dec. 31st? As far as the IRS is concerned, what is my separation date- the last day I work (12/31) or the first day I am retired (1/1)? 1/1 will be in the calendar year I turn 55, and I will not receive a lump sum until Jan/Feb 2025. Is it the arrival of the money or the date of separation that matters?

thanks

Mike