When was Saint Sebastian canonization?

St. Sebastian was not canonized. he was proclaimed a saint by early Christians because of his martyrdom. The canonization process did not come into existence until the 12th century. In the primitive Church, martyrs were immediately recognized as witnessing to the perfection of Christian life on earth, having shown the ultimate proof of their love for Christ by the offering of their lives. By the sacrifice of their lives for Christ, they attained Heaven in eternal glory and were indissolubly united to the Lord, the Head of the Mystical Body.
St. Sebastian entered heaven long before there was an official process in place for canonization. His death as a martyr was sufficent proof for the Church to declare him a saint by popular acclaim, probably with the approval of a local bishop.