If you click on the link and click on the Rashi in question, the side bar will pop up and it will say "Talmud (1)". Click on that, followed by the Avodah Zara box, to get a snapshot of that gemara. Click Open at the bottom to open it in full and learn it. If you haven't been taught how to learn gemara, find a daf yomi shiur on that page and hear it explained.
What's you'll find is that it's indeed used as an idiom, but in a different way. It's referring to the sub-category of Onah, that is not to do with pro-creation. If one is married or in chatan classes, this topic will be covered and explained.
It all starts with Reish Lakish making a controversial statement on the topic of the Golden Calf (the gemara's current theme):
אמר ר"ל בואו ונחזיק טובה לאבותינו שאלמלא הן לא חטאו אנו לא באנו לעולם
Reish Lakish says: Come and let us be grateful to our ancestors [who
sinned with the Golden Calf], as had they not sinned we would not have
come into the world.
This implies that had they not sinned, they wouldn't have had more children. The gemara attacks that assertion in several kashes, and one is:
מי כתיב (דברים ה, כז) לך אמור להם שובו לכם לאהליכם
isn't it written at Sinai: “Go say to them: Return to your tents”
(Deuteronomy 5:27)
Why would they go to their tents if they aren't going to have more children? The answer the gemara gives is that Onah in marriage isn't just about having children, but for it's own sake, which the gemara refers to as
לשמחת עונה
The enjoyment of Onah
This is cross referenced with the gemara in Pesachim which uses the term in that exact manner, proving that the mitzva of Onah isn't just about having children, and the part of it that isn't is called שמחת עונה. It's also used in this way in Yalkut Shimoni on Nach, but doesn't seem to appear in Tanach.
EDIT:
This answer has come to respond to the bounty. Now that I think about it (thanks whoever downvoted), asking a separate question in a bounty within a question is probably against the rules, which is why this answer isn't good. So allow me to add the following:
There does not appear to be any particular idiom referring to what you ask. ימי טהרה is perhaps the best we are going to get and has been agreed by 3 people in the comments, and disagreed by one.
The theory you came up with about Simchat Onah doesn't hold. As DoubleAA said, hard to prove a negative, so this is not final, although we are getting quite confident now. Please be prepared to accept that the answer to your question might be "no" and let's not try to force an answer.
