Tuesday. You can use this to shift your ancient fruits to all be on the same schedule, rather than being scattered throughout the week.
This is a key tentpole strategy for the Lazy Farmer as it really cuts down on needed trips throughout the Farm harvesting Ancient Fruit, or any long term crop like Starfruit, Cauliflower, Melon, Pumpkin, and Red Cabbage. However building up that aligned harvesting is most annoying with Ancient Fruit thanks to how you need to run your earliest Fruit through the Seed Maker to generate more. With the rest, you can just buy seeds in bulk. Just leaving some of that juicy, shiny, blue gold on the vine for up to 6 days until it's all available on the 7th is rather an exercise in self control, especially if you really need the g.
Ah, but the benefits afterwards when they all produce on the same day, and you can pop them into a Keg, which will all be
also available on the same weekday a week hence. Then to be Casked for 2 Seasons, and subsequently
also available on the same weekday. Once you have that in sync, those 3 things all happen in sequential time on the same weekday. You can even pick the day of the week so you never miss an event or birthday.
But c'mon. What's the gotcha? There's one of course, there's
always a gotcha. But it's not too big. When your weekly Harvest/Keg/Cask day happens and you have really good Luck, it does feel like a waste to spend the day on the Farm instead of tearing through Skull Cavern like a whirlwind, Fishing the Legend or whatever other thing you were saving for those best of days.
Like sneakin' off to the woods with Emily after you paid that Bear off with the sweet, sweet May-Pal Serrup.