The answer to your question is simple, but without additional context it can lead to overapplication of the same pattern in more complex scenarios where it is no longer correct to do so.
The short answer
Statics are the way to go in this case. The main reason being that this is a helper method whose result is not bound by a given context.
The long answer
The correct method depends on the goal of the method and how it achieves this.
Static methods should be used in cases where the method's purpose is not contextual and globally applicable. The logic required to find the first day of the week is always the same.
However, suppose your users are able to choose which day is the first day of the week (Sat/Sun/Mon). Now your logic to find the first day of the week is contextual and no longer globally correct (it depends on the user). While this can still be done using statics by passing the user (or their chosen day); this becomes more cumbersome when you consider more complicated example.
The better option here is to use dependency injection to inject behavior. Create multiple classes, each being an implementation of the same base class. This is a very complex topic that I can't explain from scratch, but I hope the key difference (contextuality) is clear.
There is no meaningful difference between static and instanced methods. The cost of instantiating an object is massively negligible. The only thing that can impact performance is if you have expensive logic in your constructor, but that would imply that you need this expensive logic in the constructor and thus cannot work without it, so it's still not really an issue that you should work to avoid (any more than you otherwise would).
As an aside:
Procedural code (copy and paste)
That's not procedural code. Procedural code suggests that it is generated by some tool (pun not intended), not copy/pasted by a developer.
Copy/pasted code is never the right approach, unless you can prove that these multiple instances are functionally independent of each other. In other words: they can become very different things, and the fact that they currently do the same thing is coincidence; there is no requirement that when you update one you must update all the others.
Spoiler alert: 9 times out of 10 when developers think multiple instances of the same method is the right approach, it's not.