Ice usually leaves Mendota somewhere in the second half of March, though I have seen it hang on into the second week of April and I have seen it go in February. The lake keeps its own calendar. Still, if you are trying to pick a weekend, some parts of the season are reliably better than others, and the differences are bigger than most people expect.
April and early May: cold, empty, honest
The water sits around forty five degrees, which matters less than you think until you are in it, at which point it matters enormously. Nobody swims. Everybody wears everything they own.
What you get in exchange is a lake with four boats on it and wind that behaves. Spring breeze on Mendota tends to come through steady out of the northwest, and steady is the best thing a beginner can be handed. I do most of my ASA teaching in May for exactly this reason.
June: the good month nobody books
June is the answer for most people and it is chronically underbooked, presumably because school is barely out and vacations have not started. Water is warm enough to be survivable, air is warm enough for a shirt, and the thermal breeze has started without the July haze that comes with it.
Sunset lands around half past eight, which makes an evening run possible after a workday without either of us rushing.
July and August: crowded and slow
This is when everyone wants to go out, and it is the weakest sailing of the year. Mornings die completely. The lake will sit glassy from nine until early afternoon, and there is nothing more disappointing than paying for a lesson and drifting through it.
The fix is timing rather than season. Afternoon thermals fill in reliably from about two o’clock as the land heats and pulls air across the water, so a two thirty start in August is a genuinely good sail while a ten o’clock start in August is a floating conversation. I book summer lessons in the afternoon and I say no to morning requests more often than is good for business.
Also worth knowing: July is when the algae shows up, and in a bad year the west end of Mendota gets unpleasant enough that I move lessons toward Monona.
September: the one I would pick
If somebody asked me to choose a single week to put a beginner on the water, it would be the second week of September. Air temperature is still comfortable, water is still warm from August, the powerboat traffic collapses the moment school starts, and the wind comes back as the temperature differential between land and lake sharpens again.
Fall colours around Picnic Point start turning by the end of the month, which does nothing for your sailing and a great deal for the photographs.
October: sharp, short, worth it
Strong wind, good visibility, and a real chance of a day that is too much for a first lesson. I keep sailing into November most years, but by mid October I am selective about who I take out and I cancel more often.
The last sunset run of the season is usually the third week of October, because after that the sun goes down at six and the cold arrives with it.
A note on wind, since it is the thing that actually matters
Mendota is a small lake with a city on one side and hills on the other, and that shape does strange things to air. Wind from the south comes over the isthmus broken and gusty. Wind from the northwest arrives clean across the widest fetch and is the most pleasant thing this lake offers. Wind from the east is rare and usually means weather is coming.
None of that is in a forecast app. It is the sort of thing you pick up by being out there enough times, which is a slow way to learn and the only way I know of.