Royal Mail staff are taking strike action in a long-running dispute over pay and the company's modernisation plans.
Around 115,000 postal workers are walking out for 48 hours during Thursday and Black Friday.
When do the rest of the strikes take place?
Royal Mail workers are also walking out on several other crucial days over the busy shopping and Christmas period.
More strikes are planned for 30th November and 1st, 9th, 11th, 14th, 15th, 23rd and 24th December.
How will it affect you?
Items sent the day before, during or in the days after any strike action is likely to be delayed.
So deliveries of purchases made on Cyber Monday - 28th November - could be delayed by strike action on Wednesday 30th November and Thursday 1st December.
On strike action days, Royal Mail said it will prioritise COVID-19 test kits and medical prescriptions wherever possible.
Special Delivery and Tracked 24 parcels will also be a priority - with anyone needing to send an item urgently advised to use this service.
Will I still get my letters?
No letters - except those marked Special Delivery - will be delivered, with people advised to dispatch items as early as possible.
"Items posted the day before, during or in the days after any strike action will be subject to delay," Royal Mail advise.
"We will provide further details of the expected delays to items during the period of industrial action."
No international parcels and letters will be delivered on strike days with services resuming the following working day.
Is Special Delivery still guaranteed?
Royal Mail said it will continue to accept, prioritise and deliver as many Special Delivery items on strike action days "as our resources allow".
However, delivery cannot be guaranteed by 9am or 1pm the next day for items posted from 23rd November up to and including Saturday, 3rd December, and will be suspended for this period.
Customers will not be able to claim refunds for delayed items or consequential loss due to delay for items sent on those dates.
The usual compensation rules, including those for loss or damage, will still apply during industrial action.
Royal Mail's Estimated Delivery Window will be suspended before, during and immediately after any industrial action to "avoid giving misleading information".
Text and email notifications updating customers on the progress of their item will continue.
Post offices and post boxes
Customers can continue to use post boxes during the industrial action with post offices also open and offering the usual services - but collections will be limited.
"Customers should expect delays to items sent on strike days and on the days immediately after strike action," Royal Mail said on its website.
Customer Service Points will be closed on strike days - meaning people who have requested to drop off or pick up parcels from there must wait to collect them.
Deliveries will resume the next working day after strike action concludes.
Customers should check they have received a notification confirming their item has been delivered to the service point before going there.
Royal Mail said: "After strike action, customers going to a customer service point to collect a parcel (one which has been attempted delivery or been requested to be delivered to a customer service point) should check their notifications to confirm their item is ready for collection."
Getting back to normal
Royal Mail said it wants customer services to return to normal "as quickly as possible".
"After industrial action takes place, we'll be increasing our network capacity and using additional resources to assist with getting services back to normal.
"Deliveries will also resume the day after strike action finishes."
This will include "accelerating" the delivery of NHS and healthcare letters together with "critical Government mailings".