The opposition party leader Suvendu Adhikari, on Monday, demanded that June 20th be observed as 'Poschimbongo Divas' or 'West Bengal Day' in the assembly by the state government to pay tribute to the contributions of Bharatiya Jan Sangh founder Syama Prasad Mukherjee in ensuring that the western part of undivided Bengal remained a part of India during its partition in 1947.