Navigate to Settings > Website Settings > Delivery Calendar

If your shop is usually closed and not delivering on Sundays, but you need to make Sunday, February 14th available, then

make sure Sundays are not checked, then select Sunday dates on the calendar to make them unavailable, then Save

 

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This will make Sunday, Feb 14th available, while other sundays still unavailable for delivery.

After Valentine's Day, simply check Sundays again