If it's your birthday this weekend you may not get some of your cards in time because postal workers are on strike.
Postmen and women are refusing to go to work for two days as a protest against changes to their jobs that bosses at the Royal Mail want to make.
More than 100,000 people will stop working over the two-strike.
The Royal Mail says it wants to make changes that have already been agreed to make the postal service able to cope with the future of delivering mail.
The number of letters being delivered is dropping each year, but online shopping is growing. That means the way the postal service used to work needs to change.
The postal workers have an organisation called a union that works to make sure their pay and working conditions are good as they can be. It's that union, called the Communication Workers Union (CWU), that has organised the strike.
Postal workers are going on strike all across the UK
The CWU is worried that the changes will mean lots of postal workers will lose their jobs.
Big problem for businesses
The union and the Royal Mail agree that changes to the postal service are needed, but they don't agree on how those changes will be brought in.
While missing out on post is annoying for members of the public, it can be much more serious for businesses.
Many of them use the post to deliver the things they sell, and if people can't rely on their parcels turning up on time they may start using other companies.