It sounds logical, but does one every "achieve" FI that precisely? Even if I think I need $1M to be FI and I hit that number, the market could correct and go down 30% next month. So is FI your minimal number + 30%?
Agreed. I believe a lot of people overly focus on specific calculations. They worry that FireCalc gives a 98% success rate instead of 100%, or the implications of a 3.5% SWR vs a 3.7% rate. But the gray area is huge. It isn't this precise, or more specifically, the future is not likely to be a precise replication of the past. While it is often useful to have specific goals, such as $1M in savings, it is important to realize the limitations of these goals.
The best time of year to retire can be specific to each individual. Variables include salary, retirement plan options, pension, etc. There are reasons to retire early in the year (e.g., retirement contributions, a low effective tax rate on earnings if income for the remainder of the year is low), and there are reasons to retire late in the year (save 6.2% tax free on SS contributions for people above the SS limit, paid time-off around the holidays).
While it changes by a couple of days each year, the optimal day for me to retire is around February 18. By retire I mean last day in the office, not necessarily my termination date or the date my pension starts. Beginning on February 19, I start using my 48 days of accumulated vacation (the maximum I can accumulate). I'll accumulate an extra ~5 days of vacation while taking vacation since I am still an employee. This takes me to May 5, at which time I terminate from my organization. I retain employee health benefits through the end of May and then go on COBRA for 3 months. I need to begin my pension 120 days from my termination date if I want to keep retiree medical benefits, and I need to begin my pension on the first of a month. This takes me to September 1, at which point I end COBRA, begin my pension, and start retiree medical benefits. September 1 is an optimal month for me to retire due to the way my pension works. In two of the 12 months of the year the age factor associated with the pension moves up twice as much as in the other 10 months. For me, this is September 1 and March 1.
All of this may seem quite convoluted. Most people in my situation would quit work on a given day, take accumulated vacation as a lump sum, and begin their pension on the 1st of the next month. But by extending the process as described above I increase my pension service credit, age factor, and salary base. Effectively, this is worth an extra $30K mostly due to higher pension payouts.
So for me, there is an optimal day to retire. Actually, there are two days during the year, the other being around August 18.