Thousands of people are arriving at Stonehenge in preparation for the summer solstice.
Access to the Stonehenge site is from 2200BST on Monday
The ancient Wiltshire site has drawn visitors for more than 5,000 years.
Traffic congestion leading to the stones is not expected to be as bad as the gridlock on the A303 and other surrounding roads two years ago.
Then, the solstice fell on a weekend, drawing a crowd of nearly 30,000. Police say they are ready to tow away any cars parked in the wrong place.
An English Heritage spokeswoman said: "We're expecting about 20,000. We plan throughout the year for this.
"There was about 20,000 last year and just under 30,000 in 2003."
Access to the site is from 2200BST on Monday until 0900BST on Tuesday.
BBC Weatherman Richard Angwin said: "There might be some mist around, but it should be bright enough to see the sun rise."
Stewards will be checking people for items such as glass bottles or fireworks as they enter the site.