The embarrassing gaffe was corrected Monday, less than 24 hours after Prime Minister Stephen Harper issued a news release saying voters would get a chance to fill the downtown Toronto seat on March 12.
A spokeswoman in the PMO says the new date was the result of an administrative error.
In layman's terms, a press release went out to the public announcing the byelection before cabinet had approved the date.
Elections Canada confirmed the formal order-in-council approval only arrived on Monday.
The oversight is likely to cause more irritation in NDP ranks, since it pushes the highly symbolic byelection even closer to the party's March 24 leadership weekend, when a permanent leader to replace Layton will be chosen.
Some New Democrats were already upset that Harper did not allow the party to chose its new leader before the Toronto-Danforth byelection vote was held.
Layton died of cancer last summer and the New Democrats have picked law professor Craig Scott to carry their banner in his former riding.
The Conservatives have nominated communications specialist Andrew Keyes to run for them.
They are promoting the notion that the riding is the Liberals' to lose — notwithstanding that the Grit candidate there won just 18 per cent of the vote in last May's general election.
Despite trying to drum up some star power for the byelection, only two, relatively low-profile candidates are vying for the Liberal nomination this Thursday: ad exec Grant Gordon and journalist Trifon Haitas.
Haitas is a Greek-Canadian journalist who formerly ran for the Green party and does not live in the riding.
Gordon is the founder and president of a boutique advertising firm with a quirky sense of humour. In a tongue-in-cheek pitch for the Liberal nomination, he appeals to New Democrats to support him because "I'm not a threat. The NDP owns this riding."
Gordon then appeals to Liberals by urging them to "pay no attention to that last paragraph. We can win this thing," and to Tories by asking if they "really want to see those pinko, commie, bike-riding NDPers take this riding in a walk?"
Adriana Mugnatto-Hamu, who ran for the Green Party in the 2011 federal election, was nominated by the party Sunday to run again in Toronto-Danforth.
The riding, located east of downtown Toronto, is a diverse section of the city that was known as a Liberal bastion until Layton's 2004 election. At one point in the late 1970s it was represented by then-NDP MP Bob Rae, who is now interim Liberal leader.
There are eight candidates vying to replace Layton as leader, including former party president Brian Topp, Toronto MP Peggy Nash and deputy leader Thomas Mulcair.
Layton died only a few months after he led New Democrats to official Opposition status in the House of Commons.
Note to readers: This is a corrected story. An earlier version wrongly identified Thomas Mulcair as former NDP deputy leader.
Also on HuffPost