The offering of the kathina robe started during the time of the Buddha when some monks after observing the three months rain retreat* were going to the Monastery at Savatthi where the Buddha was staying were thoroughly drenched and soaked to the skin by a heavy downpour and taking into consideration their conditions he prescribed that the "Kathina" ceremony shall be performed by monks when they have completed the "Vas". They may possess as many robes as they wanted and whatever the number of robes that the monks had receive shall belong to them.
It is the custom in Theravada countries to inform the lay devotees of the day on which the "Kathina" ceremony is to be held so that they may participate with offerings of robes sponsorship or other requisites. The distribution of the robes received by the monks from the lay devotees are entirely a matter for the sangha members. The "Kathina" ceremony promotes the interdependency of the sangha and the laity. It is an important merit making event for Buddhists following the Theravada Tradition.
* rain retreat:
This is a period of three months when bhikkhus must reside in one
place and cannot wander.
The rains residence was instituted by the Buddha to prevent bhikkhus traveling during the Rainy Season of India and S.E. Asia, and so damaging the crops, and the living creatures which are abundant then. No doubt he considered their health as well when he laid down that bhikkhus must spend the rains with four walls round them and a roof over their heads.
In Buddhist countries this is still the time for intensive activity: the meditator meditates more and undertakes more of the austere practices; the student of books makes more effort to master his studies; the teacher-monk is more active in teaching Dhamma and the writer in writing.
(Source: http://www.filmo.com/bhcrobes.htm & http://world.std.com/~metta/lib/bps/wheels/wheel206/rains.html)