Thai people are expected to make 2.44 million trips inside the country during the long holiday of July 16-20, according to the Tourism Authority of Thailand (TAT).
Total tourism spending by Thais and foreign tourists visiting the country during the extended holiday will reach an estimated 19.7 billion baht, a surge of 28% from the year-ago period.
Of the total, 12 billion baht will come from foreign tourists and 7.7 billion from Thais.
The cabinet recently declared July 18 a public holiday, giving Thais a five-day holiday period. Asarnha Bucha Day falls on July 19 and Buddhist Lent Day on July 20.
The government's announcement of a long holiday is meant to encourage Thais to travel inside the country and boost tourism business sentiment.
TAT governor Yuthasak Supasorn predicted the Central region would generate the most tourism income during the holiday period at 3.16 billion baht, followed by the East at 1.28 billion, the South at 1.26 billion, the North at 1.06 billion and the Northeast at 914 million.
During the five-day holiday, average hotel occupancy is expected to reach 61% nationwide.
The Central region will see an occupancy rate of 70%, the East 61%, the North 49%, the Northeast 57% and the South 62%.
The top five destinations during the holiday are Bangkok, Chon Buri, Nakhon Ratchasima, Ayutthaya and Kanchanaburi, the TAT said.
Apart from Thai tourists, the TAT forecasts 1.09 million international tourist arrivals to Thailand during the five-day holiday, generating 12 billion baht in revenue, up 28% for both figures.
Mr Yuthasak said the TAT has continually launched tourism activities every month to woo Thais to travel inside the country.
With the current economic situation, the TAT has accepted that it will miss this year's revenue target for domestic tourism of 900 billion baht, putting the likely figure in a range of 830-850 billion baht.
The TAT recently cut its 2017 domestic tourism revenue target from 1 trillion baht to 950 billion because of concerns about the economic downturn.
The Thai Travel Agents Association (TTAA), meanwhile, said outbound tourism would grow at a single-digit rate this year, hampered by a slowdown in the third quarter.
To this point, TTAA president Suparerk Soonrangura expects intense competition among travel agents in the third quarter as they capitalise on a good opportunity for customers to travel abroad.
The association is confident that many Thai travellers will use the extended holiday to travel abroad, especially to short-haul destinations.
The top five outbound destinations for Thai travellers are Japan, Singapore, mainland China, Hong Kong and South Korea.