From what know, every countable word needs to have an article. If I would write the sentences below, I would use the definite article after each "of" word.
.. end of the every sentence... end of the each month.
Put a period at the end of every sentence.
I pay the phone bill at the end of each month.
I know that there are some exceptions by generalizing or when a word is representing some class but I don't think that this is the case.
Why the author of these sentences just omitted the definite articles?