My work pays their bonus in April. My husband's work pays their 401K match, lump sum, in April. The rules on both are that the money is earned the year prior, but you have to be employed at the date of payout. My employer makes quarterly contributions to my HSA as well, with one being in April, so working through April is a given for us with all those things lining up.
We're hoping a retire date in April allows us to front-load our 401K and HSA to try to reach the contribution limits already for the year. Also, with the extra income in those first few months, hopefully we can max out our Roth IRAs as well. Basically, most of the income in those first few months will be "spoken for" in terms of investing, which is why I think it's nice to plan on working at least a few months into whatever year you plan on retiring. It gives a nice boost to savings.
One other thing to think about is if there is a 3 paycheck month coming up. I get paid every 2 weeks, so ending on a 3 paycheck month with a nice bump to that month's savings has it's appeal. The years we're looking at retiring, that isn't until July though, so I might not hold out that long.
In terms of vacation accruals, our employer has a very generous cap on vacation accruals (~300 hours or something crazy). People who retire around here tend to just cut down on their hours and spend their vacation time actually taking vacations rather than taking the lump sum payment. This allows them to ease out of working, train in their replacement, still be covered by insurance the entire time, and still ACCRUE vacation while they work here. I can see the benefit in just cutting the cord, but for folks that are actually at retirement age, they get a lot less shit for retiring than a younger person and management is a lot more receptive to allowing them to cut down to 3-4 days a week or working during busy times only until their vacation is used up. It seems to benefit both the retiree and management. HR doesn't like it, but that just means people stopped asking HR... ha.
Lastly, if you have a Flex Spending Account (and if the rules haven't changed since the last time I had a Flex Account) money is funded 100% on day 1 of the year. You could potentially bump up your Flex contributions to the maximum for the year and spend the money immediately before retiring and get your last blast of medical/dental work done for "free." This might not be a moral way to approach this, but plenty of people have been on the reverse end of the "use it or lose it" policy and lost money by contributing more than they spend. If you've lost money in the past, consider this your chance to get it back. Conversely, make sure you actually spend the money, you've contributed before you retire. Definitely correct me if I'm wrong on this - it's been a long time since I've had a Flex. I love my HSA so much more.
I agree with spring being a nice time for non-monetary reasons as well... having nice weather and plenty of free outdoor recreation to keep you busy, having more daylight for biking, and being able to take on gardening if that's your thing all sound like great ways to keep busy.