Every after 21 Days job expression

I want to run a hang fire job every Monday after 21 Days job expression. Please suggest me expression.
**ex 17th July 1AM Monday 2023 , 7th August 1AM Monday 2023 , 28th August 1AM Monday 2023 and so on...**

The issue names no file, test, or entry point. First clarify whether it requests a scheduling expression or a product change, then identify the existing recurring-job scheduling entry point and verify whether the Monday, 21-day sequence can be represented. Done means providing a confirmed expression or a clearly scoped implementation request.

Written by the indexing model from the issue text.