If you've been selected for jury service you will receive a summons.
Your jury summons will state when your jury service will take place.
You can request to defer your jury duty until a more convenient time. If you apply for a deferment, you can only do this once, so you must inform the courts of any other times over the next 12 months when you definitely won’t be available. They can then make arrangements for your jury service around your available dates.
As well as a deferment you can also apply to be completely excused from your jury duty. To be totally excused jury duty you have to have a very good reason, supported with evidence (such as if you have served on a jury within the previous five years).
If the court does not accept your request then you are obliged to serve as a juror. Once your jury service is confirmed, you must turn up on the agreed start date and if you fail to attend you could be fined £1,000.
See Gov.uk for more information.