The Daughters of Israel Took It Upon Themselves
The Raavad describes how the women of Israel added to the counting of clean days on their own, and how that custom became the law for all.
In his laws of counting and examination, the Raavad explains the story behind today's practice. In earlier times women were expert in telling one kind of blood from another. As the generations passed and understanding faded, Rebbi enacted a rule for the villages, so that daughters of the countryside would not confuse the different days. Even after that rule, he writes, the daughters of Israel accepted a further stringency on their own. They counted seven clean days even for a single day of seeing. So today every woman counts as one who must count seven clean days, while her immersion follows the law of a woman in her days of separation. The Raavad treats the voluntary strictness of Israel as something weighty.
Source: Baalei HaNefesh, Shaar HaSefirah VeHabedikah, siman 1, paragraph 5
The women of Israel were not commanded to add this stringency, yet their devotion changed the practice of the whole nation. You do not need to wait for a rule to add something good to your life. Choose one small act of your own, such as a short chapter of Tehillim each morning, a fixed time to learn, or a call to a lonely relative each week. Start this week, keep it simple, and hold on to it.