The union said more than 100,000 workers are involved, making it the biggest strike of the summer.
The action follows a walkout last week and there will be further stoppages on Thursday September 8 and Friday September 9.
The action is in protest at a 2% pay rise, although the company has said more money is on offer.
CWU general secretary Dave Ward said: “There can be no doubt that postal workers are completely united in their determination to secure the dignified, proper pay rise they deserve.
“We can’t keep on living in a country where bosses rake in billions in profit while their employees are forced to use food banks.
“Postal workers won’t meekly accept their living standards being hammered by greedy business leaders who are completely out of touch with modern Britain.
“They are sick of corporate failure getting rewarded again and again.”
CWU deputy general secretary Terry Pullinger said: “Our members worked miracles during the pandemic and know full well what they are worth.
“They are fighting for a no-strings, real-terms pay rise – something they are fully entitled to.
“Those managing Royal Mail Group are treating our members with contempt by imposing such a minimal amount.
“Royal Mail Group have failed to recognise the strength of feeling and have clearly lost the dressing room on pay, so they have left us with no choice but to fight.
“Our members deserve a pay rise that rewards their fantastic achievements in keeping the country connected during the pandemic, but also helps them keep up during this current economic crisis.
“We won’t be backing down until we get just that.”
A Royal Mail spokesman said: “The CWU’s self-centred actions with the wider trade union movement is putting jobs at risk, and making pay rises less affordable.
“We are losing £1 million a day and the CWU’s strike action is making our situation worse. We want to protect well-paid, permanent jobs long term and retain our place as the industry leader on pay, terms and conditions.
“Each strike day makes that more difficult, making Royal Mail’s future more uncertain than at any time in its long history.
“The CWU has failed to respond to our latest invitation to meet to discuss change and pay, instead creating red herrings on the Universal Service, renationalisation and shareholder activity. The CWU is deflecting to avoid talking about the changes we need to make as a business.
“On the CWU’s first strike day, more than 850 offices were operational as we worked to minimise customer disruption and keep people, businesses and the country connected.
“Over the Bank Holiday weekend, teams across Royal Mail have worked tirelessly to implement our recovery plans, ensuring NHS letters and critical government mailings were prioritised as we cleared the mail to return to normal service levels.
“We need the same commitment from the CWU’s leadership to engage on change, this is the only way we can unlock more pay.
“Our future is as a parcels business. We must adapt old ways of working designed for letters to a world increasingly dominated by parcels and act fast.
“We cannot cling to outdated working practices, ignoring technological advancements and pretending that Covid has not significantly changed what the public wants from Royal Mail.
“We apologise to our customers for the inconvenience the CWU’s strike action will cause.
“We remain ready to talk with the CWU to try and avert damaging industrial action and prevent significant inconvenience for customers. But any talks must be about both change and pay.”