The Election Commission of Pakistan (ECP) had earlier decided to hold the elections on March 3 but it was delayed by two days to spend more time on the scrutiny of the candidates.
Under the law, the candidates should fulfill the requirements under Articles 62 and 63 of the Constitution and their papers are scrutinised by the ECP.
The Senate is a continuous body and the Senators are elected for six years but under the law, half of the 104-member House retire after every three years.
All four provinces have equal representation in the Senate to make it true symbol of a federation.
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Those retiring include majority 21 from former president Asif Ali Zardari's Pakistan People's Party (PPP), which will lose its majority in the upper house.
As the new senators are elected by the provincial lawmakers, ruling Pakistan Muslim League-Nawaz of Prime Minister Nawaz Sharif will get a major boost in the new Senate but it will remain short of majority and will need support of smaller parties to get its legislation through the Senate.
PPP will elect most of the senators from Sindh province while PML-N will get its nominees elected through the Punjab assembly.