In the UK the clocks go forward 1 hour at 1am on the last Sunday in March. In 2017, this is the 26th of March.
They go back 1 hour at 2am on the last Sunday in October.
It is useful to remember the old saw “Spring forward, Fall back.”
The period when the clocks are 1 hour ahead is called British Summer Time (BST). There’s more daylight in the evenings and less in the mornings (sometimes called Daylight Saving Time).
When the clocks go back, the UK is on Greenwich Mean Time (GMT).