Buddha's Dispensation (Buddha-sasana) is the time period which Dharma (Pali Canon) exist and follow. It is totally 5000 years after the Buddha's Parinirvana (pass away or "Skandha Parinirvana"). There are three types of Parinirvana those are "Klesha Parinirwana", "Skandha Parinirvana" and "Dhatu Parinirvana".
Dhatu Parinirvana will occur the end of the Buddha's Dispensation (totally 5000 years after the Buddha's Skandha Parinirvana). It happen according to the Buddha's wish even he doesn't exist. All Buddhas Relics in the Ruwanwelisaya (the place where large amount of Buddhas Relics exist) in Sri Lanka will together and make a picture of a Buddha in the sky above Ruwanwelisaya. Then those relics and all Relics associated with Buddha exist in the human world, Naga world, heaven words and Brahma worlds will came to the sky above the Vajrasana (diamond throne) in 'Bodh Gaya' of India and will make a complete Buddha's picture and preach Dharma for the last time. This will not be a normal preach like in the 'Sutta Pitaka' in Pali Canon, it will be a 'Samukkansika mode of preaching' extensively on 'Four Noble Truths' which preach with Buddha's super-normal abilities. This is a very long (containing many hundred thousand words) preach, do in high speed but every one can understand each word clearly. With this preach, many beings realise Nirvana. Then Vajrasana will come to the sky from the earth and all Buddha's Relics and Vajrasana will completely burned out without any ash.
After this 'Dhatu Parinirvana' and when the last Sotāpanna human passed away from the earth it is said to be the end of Buddha's Dispensation. Before the end of Buddha's dispensation any one who correctly follow Buddhas words it is said to be able realize the Nirvana. But after the Buddha's Dispensation it seems it is impossible since no one has energy to correctly follow Buddhas words. But if any great being correctly follow Buddhas words even after the Buddha's Dispensation it is said to be able to realize the reality (Nirvana).
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