It is the first of four days of industrial action, with walkouts also taking place on 31 August and 8 and 9 September.
Letters will not be delivered on strike days and some parcels will be delayed, Royal Mail warned.
The union representing the workers is demanding a pay rise that more closely reflects the current rate of inflation.
Royal Mail apologised to customers and said it had contingency plans to minimise the disruption.
On strike days it will deliver as many Special Delivery and Tracked 24 parcels as possible, it said. It will also prioritise the delivery of medical prescriptions where possible.
However, it said items posted the day before a strike, during the strike or on the days after may be delayed.
The company is encouraging people to post items as early as possible to avoid disruption.
Businesses which use Royal Mail have also issued warnings to customers.
The card company Moonpig has advised customers to order early where possible but said its gifts and flowers use different delivery services so would be unaffected by the strikes.
The flower firm Bunches said it would send goods using DPD's next day courier service for a reduced price on strike dates.
It comes as Royal Mail said the Communication Workers Union (CWU), which represents the strikers, had rejected a pay rise offer "worth up to 5.5%" after three months of talks.
The union has called for Royal Mail to increase wages to an amount that "covers the current cost of living".
Inflation, the rate at which prices rise, is at a 40-year high of 10.1% and expected to peak at 13% later this year.
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."
He added: "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.
"When Royal Mail bosses are raking in £758m in profit and shareholders pocketing in excess of £400m, our members won't accept pleads of poverty from the company."
Royal Mail's latest adjusted operating profit for the year to March was £416m, up from £344m a year earlier.
A Royal Mail spokesperson said the business could not "cling to outdated working practices, ignoring technological advancements and pretending that Covid has not significantly changed what the public wants from Royal Mail".
"While our competitors work seven days a week, delivering until 10pm to meet customer demand, the CWU want to work fewer hours, six days a week, starting and finishing earlier," they added.
"The CWU's vision for Royal Mail would create a vicious spiral of falling volumes, higher prices, bigger losses, and fewer jobs."
The company said it remained ready for further talks to avert the strikes, but that they "must be about both change and pay".
Chairman Keith Williams has said the firm is losing £1m a day as parcel volumes fall and efforts to modernise the business stall.