Special light offerings are also made on auspicious
occasions. On full-moon days when devotees flock to
the temples, lamps are lit in large numbers, for it is
the custom among the Sri Lankan Buddhists invariably
to take flowers and coconut oil on their visits to the
temple as two indispensable articles of worship. There
are also occasions when devotees light and offer a
particular large number of lamps for special purposes,
such as redeeming a vow (baraya) or on special
occasions like Vesak Day. Many Buddhists perform the
ritual of light offering (pahan-puja) to counter evil
planetary influences. In order to obtain maximum
results from the ritual, the devotees make it a point
to purify themselves completely before attending the
ceremony by bathing and wearing fresh, clean clothes.
Coconut oil used as an illuminant is specially
prepared for the purpose and taken separately from the
coconut oil used for household purposes. Wicks are
prepared from a clean, white, fresh cloth. Sometimes
the inhabitants of an entire village co-operate in
holding a mass-scale lamp offering. For instance, they
may offer 84,000 lighted lamps in memory of the 84,000
elements of the Dhamma (dhammakkhandha) comprising the
Buddha's Teaching.
http://world.std.com/~metta/lib/bps/wheels/wheel402.html#ch3