meaning of "the second Sunday backwards"

[All Potted Out]

Is it correct to say "The shop's closed days are the second Sundays by counting backwards from the end of each month"? Take this coming March for example, the shop's closed day should be the 18th.
Is there a better or shorter way to express this?

[Mister Micawber]

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Well, it seems like a ridiculous system to me, but I would say 'The shop is closed the second-to-last Sunday of each month.'
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Native English teacher at Mister Micawber's