Critics accuse Nkurunziza of trying to stay in power for life and say a personality cult has developed around the former rebel chief who has led the central African country since 2005.
The government in October drafted reforms that would enable Nkurunziza to serve two seven-year mandates from 2020, but they have come under fire from the opposition and the international community, particularly the African Union.
The opposition says the changes could sign the death warrant for the Arusha peace accord of 2000, which helped end a 1993-2006 civil war that claimed more than 300,000 lives.
Nkurunziza ran for a third five-year term and was re-elected in 2015 despite a two-term limit under the constitution, triggering violence that left at least 1,200 people dead and sent more than 400,000 Burundians fleeing abroad.
The government had previously announced that the referendum would take place in May but had not announced the exact date.
The reforms will be adopted if 50 percent plus one vote cast ballots in favour.
Sunday's decree said those wanting to take part in the official campaign must register with the Independent National Election Commission (CENI) between March 23 and April 6, but no-one is yet allowed to publicly declare themselves for or against the reforms.
The opposition has denounced what it describes as double standards, saying that ministers and officials with the ruling CNDD-FDD have openly campaigned for a Yes vote, while several dozen opposition activists have been arrested for pushing for a No vote.
The election commission announced last month that more than five million people had registered for the referendum and for the next election in 2020.
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![Graça Machel was born in 1945 [CGTN]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'300'%20height%3D'172'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Graça Machel was born in 1945 [CGTN]")
