Hello, @slowmart! A few things (bear with me, I’m an RD and have many thoughts on your question):
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If this is a one-time deal, you’ll be fine. However, making this large Saturday deficit a weekly habit could generate a few negative consequences, including overeating later in the week, muscle loss, and suboptimal Sunday rides.
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Try to nail your pre-, during, and post-nutrition as much as possible for that Saturday ride. I’d say to definitely not skimp on the immediate post-ride nutrition to replenish glycogen stores. Can you throw in a big snack after dinner? Something like a smoothie/shake, big muffin, or trail mix would do the trick.
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Don’t forget the protein post-ride, and in general - going too hard on the weekly calorie deficit will just lead to muscle loss…no good.
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All of that being said…calorie estimates on nutrition are just that - estimates. Food labels can be off by about 30%, and expenditure estimates can vary widely, even with HRMs. So, see how you are feeling on Sunday’s ride, track your weight loss over time, and get your body comp checked as you lose weight.
Hope that helps 