Process Builder - Update on first of each month

Having a hard time wrapping my head around this request and would appreciate ya'lls help!

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I need to have an automated process (hopefully via Process Builder/Workflow since I can't code) so that:

- On the LAST day of every month, a field on every open opportunity is updated with a value.

- The value is that is updated in the field is simply copying the value from a separate formula field on the opportunity.

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Think of it as every last day of the month, a formula field on the opp is copied over and hardcoded onto a text field.

Anyone have tips on how to manage that request?

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EDIT: My post title says "first" but really I need the last day of each month