Nisaa
Hayd occurs when women are going through menstrual bleeding. Menstrual blood has distinct characteristics: it is dark in color, thick in consistency, and has a distinct odor different from ordinary blood. The Prophet (Salla Allahu Alayhi Wa Sallam) said to Fatimah bint Abi Hubaysh: “When it is menstrual blood it is dark and recognizable, so when it is like that, then stop praying, and when it is otherwise, do wudu and pray, for that is from a vein.” (Narrated by Abu Dawud and al-Nasai; classed as sahih by Ibn Hibban and al-Hakim). If you observe this type of blood, it signifies your menses. Clear or white discharge that increases in the days before bleeding starts is a different thing and does not signal hayd on its own; see why discharge increases before your period for what is actually happening then. Light spotting in the day or two before bleeding starts is also a different thing from hayd itself; see why you might notice spotting before your period for why a trace of blood beforehand does not settle the question on its own. The same goes for headaches, tiredness, mood swings, breakouts or other premenstrual symptoms that show up in these same days: none of them are the blood itself; see why headaches happen before your period, why mood swings happen before your period and why breakouts happen before your period for the hormonal reasons behind them. A period that hasn't started at all, later than you expected it, is a different question again; see why your period might be late for the usual reasons it can shift.
Regarding the duration of period, scholars differed.
Some scholars considered that there is no minimum or maximum number of days for Hayd, as there is no clear evidence in the Quran and Sunnah specifying a limit, as long as there is nothing indicating that it is Istihadah. This was the view of some scholars such as Sheikh al-Islam Ibn Taymiyyah.
Other scholars have defined a minimum and/or maximum number of days for Hayd based on customary practice and observed experiences of women, though these limits vary across the four madhhabs.
Minimum duration:
- Malikis: no set minimum (even one gush).
- Hanbalis & Shafi'is: one day and one night.
- Hanafis: minimum three days.
Maximum duration:
- Hanafis: ten days.
- Shafi'i, Maliki, Hanbali: fifteen days. Anything beyond that is Istihadah.
Malikis also differentiate between a girl’s first-ever period and women who have already menstruated before. For the first period, up to 15 days can be considered Hayd. For later cycles, the maximum is her previously known longest Hayd + 3 days, not exceeding 15 days.
Most scholars agree that menstruation generally does not exceed 15 days, and the period of purity cannot be shorter than menstruation.
Once hayd ends, ghusl is what makes you pure again to pray, not the stopping of blood on its own.
Many women find it easier to keep these characteristics straight month to month with a period tracker built for Muslim women, one that logs what the blood actually looked like rather than just a date on a calendar.