The Rains Retreat
The bhikkhus year is structured around the three months from July to October. In Asia this is the time of the monsoon season — the central period of the agricultural year — when the paddy fields are flooded and the main rice crop is planted. In the Buddhas time (and until modern times), people were less likely to travel around during this period because the roads were bad and there was a danger of crop damage. So the bhikkhus likewise suspended their mendicant wanderings and had to settle in one place.
A bhikkhu must make a formal determination to be resident at dawn every day in that place for the whole three month period. (There are exceptional circumstances when he may be allowed to be away, but even then he should return within seven days.) These three months are often a special time of study or meditation and so are sometimes known as the Rains Retreat or Rains Residence. This is also the normal time when the young men of South East Asia become monks for the traditional three month period (see above).
A bhikkhu often measures the length of time he has been a monk according to how many Rains Residences he has undertaken. Therefore instead of saying he has been ordained seven years he might say he has been ordained for seven Rains.
This special leave of absence (sattaaha) can only be taken in order to: visit or nurse ill Dhamma friends and parents; support fellow bhikkhus who are thinking of disrobing; to attend to some essential duty of the Community; to support faithful lay devotees who make an invitation. (See EV,II,pp.84; 89-90)