It was not a one-off, but one of several where they enjoyed the advantage towards the end of 2020.
Little was going in Boris Johnson's favour at that point - the country was entering another lockdown after months of argument about the tiers system in England, and the COVID-19 vaccine rollout had only just begun.
Everything has changed. This week, Labour is 11 points behind the Conservatives.
Sleaze allegations have done little to nudge the numbers in their favour. Labour tempers are starting to fracture, and the aftermath of next week's elections in England, Scotland and Wales could be crabby.
A Sky News analysis of the demographic and voting shifts displayed in YouGov voting intention data over the last four months reveals the scale of the challenges for Sir Keir and his party.
On entering office last April, Sir Keir had two principal electoral challenges in England.
Firstly, he had to re-unite voters on the left, which fractured in several directions after four years of Jeremy Corbyn's as leader.
Then he had to begin the altogether tougher task of winning over Tory votes.
By December, it appeared that Sir Keir had achieved the first of these tasks - reuniting the left - and that allowed him to edge ahead.
He was holding on to almost all the electoral coalition Mr Corbyn had assembled, with 88% of Labour voters in the 2019 election sticking with the party.
He was also attracting people who voted Lib Dem in the 2019 general election in big numbers - 54% of Lib Dems said they were planning to switch to Labour.
A solid majority of Remainers - 58% - said they would vote Labour.
Now, at the end of April, Labour is losing many of these advantages. The proportion of people sticking with Labour has dropped from 88% to 81%.
This seven point drop is a significant number - it could represent over 800,000 people who voted Labour in 2019 deserting the party.
There is no one destination for these votes, the data suggests - and they are fairly evenly spread across all of the other political parties.
The Tories are doing much better on this metric, currently holding on to 87% of their voters from 2019.The data also shows a significant number of Lib Dem voters who in December were flirting with Labour have returned to the party they voted for in 2019.
The proportion prepared to make the switch dropped from 54% in December to 32% now.
After Sir Keir's party walked through the division lobbies with Tory MPs in support of Mr Johnson's Brexit trade deal at the end of 2020, it appears that now only 48% of Remain voters would chose Labour, down from 58% in December.
Through the autumn it did not appear Labour were picking up Tory voters, with less than 10% switching between the two big parties.
This remains true today, and could be a warning why 'Red Wall' areas will not fall readily back to Labour hands in council and mayoral elections next week.
Elsewhere, the numbers show other challenges for Labour, specifically the effect of the coronavirus vaccine rollout.
In December, people aged 50 to 64 were evenly split between Labour and Tory, with 37% giving their vote to each party.
Now this demographic - all of whom have had at least one vaccine jab - has swung firmly in favour of the Tories, with 50-64 years choosing the Tories by 50% with only 30% opting for Labour.
Sir Keir's party has almost entirely erased their 26 point lead over the Tories amongst 25-49 year olds last December.
Now 39% would vote Labour amongst 25-49 year olds, while 37% would vote Tory in a group where only a small proportion have had one vaccine jab, suggesting this demographic has the potential to shift even more heavily against Labour in coming weeks.
Labour still maintains a clear advantage amongst 18-24 year olds, although turnout in this group is often low.
The speed of the drop in Labour's poll numbers appears to have shaken the party, although what can be lost this fast may be able to be repaired at speed as well.
However, Sir Keir appears to be going backwards, needing to shore up after Remainers on the left, meaning the job he has barely begun of winning back Tory votes looks harder than ever.