Question about words which have 2 genders

I’m interested to know why some words have 2 genders, which are not “une femme” or “un homme” or “une chien” or “une chienne”, but the words like “le jour” and “la journée” or “le soir” or “la soirée”.

It has to do with units of time and duration of an ‘event’ (année, journée; soirée)

Here is one of many links I have found which will explain it a bit better : http://www.pomme.ualberta.ca/Pomme/FMPro?-DB=R.fp5&-Format=regle_frame.htm&-lay=default&-op=eq&ID=1524&-find

what a great link-- thanks!

Thank you, SanneT!