Workers will stage five days of strikes from Monday in a dispute over jobs, pensions and branch closures.
The industrial action will affect Crown Post Offices - the larger branches usually located on High Streets.
But Dave Ward of the Communication Workers Union said it was "now or never". "[We have to do this] if we want to make a difference," he said.
"We already know that the government and the company are going to announce a further tranche of closures in January,"  he told the BBC's Today programme.
The Communication Workers Union (CWU) said the walkout would start on Monday 19 December and include Christmas Eve.
But the Post Office said despite the walkout it would be "business as usual" with "at least" 97% of its 11,600 branches not involved.
A spokesperson from Royal Mail said: "If any industrial action takes place in Post Office Ltd, Royal Mail will be operating as normal. We expect there to be no or little effect on the services we provide to our customers.
"Royal Mail customers will also continue to have access to Royal Mail services including pre-paid parcel drop-off through over 1,200 customer service points at delivery offices nationwide."
In April 2016, the Post Office announced plans to transfer up to 61 branches into WH Smith stores over the following year.
It said the move was part of a 10-year plan to cut costs and save cash, and would act as a way of "safeguarding the future of the network."
"Our members want the Post Office management to pause its closure and privatisation programme, hold off on its planned pensions changes, and commit to sitting down with us and with the other key stakeholders of this Great British institution and, together, construct a lasting vision," said CWU assistant secretary Andy Furey.
The Post Office said it was "extremely disappointed" by the CWU's action.
"Just today, we agreed with the CWU that we would resume talks, which have been ongoing throughout the summer, on Wednesday," added Kevin Gilliland, the Post Office's network and sales director.
A union representing Post Office workers has defended a decision to strike in the week before Christmas.