After spending four vassas (residence period during the rains) after his Enlightenment, the Buddha visited Kapilavatthu, his native royal city, at the request of his ailing father, King Suddhodana. At that time, Mahapajapati, Buddha's foster mother requested him to admit her into the Order. Mahapajapati was not alone in desiring to join the Order. Five hundred Sakyan ladies whose husbands had left the household life were also eager to be admitted into the Order
After his father's death, the Buddha went back to Vesali, refusing the repeated request of Mahapajapati for admission into the Order. The determined foster mother of the Buddha and widow of the recently deceased King Suddhodana, having cut off her hair and put on bark-dyed clothes, accompanied by five hundred Sakyan ladies, made her way to Vesali where the Buddha was staying in the Mahavana, in the Kutagara Hall.
The Venerable Ananda saw them outside the gateway of the Kutagara Hall, dust-laden with swollen feet, dejected, tearful, standing and weeping. Out of great compassion for the ladies, the Venerable Ananda interceded with the Buddha on their behalf and entreated him to accept them into the Order. The Buddha continued to stand firm. But when the Venerable Ananda asked the Buddha whether women were not capable of attaining Magga and Phala Insight, the Buddha replied that women were indeed capable of doing so, provided they leftjthe household life like their menfolks.
Thereupon Ananda made his entreaties again saying that Mahapajapati had been of great service to the Buddha waiting on him as his guardian and nurse, suckling him when his mother died. And as women were capable of attaining the Magga and Phala Insight, she should be permitted to join the Order and become a bhikkhuni
The Buddha finally acceded to Ananda's entreaties.
"Ananda, if Mahapajapati accepts eight special niles, garu-dhamma, let such acceptance means her admission to the Order "
The eight special rules  are:
(1) A bhikkhum, even if she enjoys a seniority of a hundred years in the Order, must pay respect to a bhikkhu though he may have been a bhikkhu only for a day
(2) A bhikkhum" must not keep her rains-residence in a place where there are no bhikkhus.
(3) Every fortnight a bhikkhuni must do two things: To ask the bhikidiu Samgha the day of uposatha, and to approach the bhikkhu Samgha for instruction and admonition.
(4) When the rains-residence period is over, a bhikkhuni must attend the pavdrand ceremony conducted at both the assemblies of bhikkhus and bhikkhun*s, in each of which she must invite criticism on what has been seen, what has been heard or what has been suspected of her.
(5) A bhikkhuni who has committed a Samghadisesa offence must undergo penance for a half-month, pakkha manatta, in each assembly of bhikkhus and bhikkhunis.
(6) Admission to the Order must be sought, from both assemblies, by a woman novice only after two year's probationary training as a candidate.
(7) A bhikkhuni should not revile a bhikkhu in any way, not even obliquely.
(8) A bhikkhuni must abide by instructions given her by bhikkhus, but must not give instructions or advice to bhikkhus.
Mahapajapati accepted unhesitatingly these eight conditions imposed by the Buddha and was consequently admitted into the Order.
Footnotes and references:
Vide, Vinaya-II, 74-75