The island's two main monsoon seasons run from May to September and December to February.
The victory celebrations scheduled for Thursday were put off as the military was unable to go ahead with rehearsals due to torrential monsoon rains, a defence official said.
Sri Lanka declared victory after the killing of the top leadership of the Tamil Tigers on May 18 last year following 37 years of bitter conflict which claimed more than 100,000 lives according to United Nations estimates.
The authorities had scheduled a military parade on Thursday to mark the defeat of the Liberation Tigers of Tamil Eelam (LTTE) and the ending of their campaign for an independent state for Sri Lanka's minority Tamils.
Officials said military preparations on Monday were washed out due to heavy rains which also killed at least four people and drove 200,000 people out of their flooded homes.
The Meteorological Department warned there could be landslides in some parts of the country.
Since Sunday, electricit