Postal workers astatine Royal Mail person begun a question of strikes successful the run-up to Christmas successful a enactment implicit wage and conditions.
The walkouts involving 115,000 workers volition deed deliveries crossed the UK, Royal Mail said.
Millions of pieces of message person been piling up up of the action, the CWU national says.
Negotiations betwixt the national and Royal Mail person breached down.
Members of the CWU national are owed to onslaught connected 9, 11, 14, 15, 23 and 24 December.
Last week customers were advised to station Christmas message earlier than accustomed owed to the strikes.
The national has said its members privation a wage emergence that matches the soaring outgo of living, and that members consciousness their absorption wants to crook Royal Mail successful to a gig system firm, akin to Uber.
It warned of a "Christmas meltdown" successful parcel and missive deliveries, and said millions of items of message had been stacking up earlier the strikes.
The national said its members were facing "massive real-terms wage cuts" and that absorption wanted to "force done thousands of compulsory redundancies".
Dave Ward, CWU wide secretary, said: "Royal Mail bosses are risking a Christmas meltdown due to the fact that of their stubborn refusal to dainty their employees with respect.
"Postal workers privation to get connected with serving the communities they beryllium to, delivering Christmas gifts and tackling the backlog from caller weeks.
"But they cognize their value, and they volition not meekly judge the casualisation of their jobs, the demolition of their conditions and the impoverishment of their families."
Talks person breached down betwixt the national and Royal Mail, a spokesperson said, adding that Royal Mail managers are "refusing to budge with their 'best and final' offer".
That connection includes a 9% wage woody implicit 18 months and "a fig of different concessions to presumption and agreements", Royal Mail said.
A Royal Mail spokesperson said: "We spent 3 much days astatine [conciliation service] Acas this week to sermon what needs to hap for the strikes to beryllium lifted.
"In the end, each we received was different petition for much pay, without the changes needed to money the wage offer," the spokesperson said, adding that the national "knows afloat well" that the concern is losing much than £1m a day.
He added that the onslaught enactment had outgo unit £1,200 each. "The wealth allocated to the wage woody risks being eaten distant by the costs of further onslaught action," helium said.
The spokesperson added that CWU was "deliberately holding Christmas to ransom for our customers, businesses and families crossed the country".
He said Royal Mail was "doing everything we tin to present Christmas for our customers and settee this dispute" by continuing deliveries, but "the task becomes much challenging arsenic Christmas nears".
The quality began this summertime aft Royal Mail rejected national demands for a wage emergence that matched ostentation - the complaint astatine which prices rises - which is presently 11.1%.
Royal Mail has been struggling arsenic it moves from its accepted concern of delivering letters - which is nary longer profitable - to the fast-growing satellite of parcel deliveries.
The institution has announced plans to chopped up to 10,000 jobs.