The Prince of Wales Bridge will close westbound on the penultimate weekend before Christmas to remove the M4 toll booths as the bridge goes toll free.
The renamed Second Severn Crossing will shut westbound on Friday, 14 December at 20:00 GMT ahead of the Severn tolls being scrapped on Monday, 17 December.
Christmas shoppers and Cardiff fans returning from their Premier League game at Watford will be affected.
Motorists will be diverted around the original M48 Severn Bridge.
The M4 bridge will reopen early on 17 December for a formal ceremony, as it is believed it will be the first time in about 400 years that crossing the Severn estuary will be free.
These will be narrow lanes with a 50mph speed limit.
The M48 crossing between Aust in Gloucestershire and Chepstow will be closed westbound later on 17 December, reopening early on 19 December so the booths on the English side can be taken away.
Further work will be carried out in 2019 to return both routes to a three-lane motorway with the usual 70mph speed limit.
Vehicles have had to pay to cross between south west England and south Wales since the first bridge was opened by the Queen in 1966
Regular users of the bridges, which costs £5.60 for a car travelling westbound, could save as much as £1,500 a year.
Drivers using both bridges have been warned to expect disruption and speed restrictions until the work on both bridges is completed.
About 25 million journeys a year are made across the two bridges and the Welsh Government estimates the abolition of the tolls will give a £100m boost to the Welsh economy.
The toll on the bridges was initially reduced on New Year's Day in 2018 after they returned to public ownership as the UK government removed VAT on their take-over.
Severn Bridges General Manager, Hannah Milliner, said: "We would like to thank drivers in advance for their patience while we carry out this work and to ask them to plan ahead for any journeys they are considering."
Wales' First Minister Carwyn Jones called for the toll, which is worth up to £10m a month for the government, to be scrapped straight away on 1 January 2018.
But the Department for Transport said the fees collected in 2018 would help pay to phase out tolling and pay towards the estimated annual maintenance and operational cost bill of £15m.
The Welsh Government said the toll removal "will help provide greater opportunities for those looking to visit and trade in and with Wales".
The Queen opened the £8m first bridge in 1966 while the second bridge, built three miles downstream across the Severn Estuary, was financed by a private consortium set up in 1992.
The newest bridge, opened in 1996, cost £332m to construct but the eventual repayments including debt repayments, interest and tax totalled more than £1.3bn.
Severn River Crossing's 180 staff were transferred to UK government agency Highways England on 1 January 2018 but with tolls set to be scrapped, 100 toll collector and administration jobs are going.