The following are national days in Thailand though none of them are celebrated as public holidays.

National Children's Day - 2nd Saturday of January วันเด็กแห่งชาติ (Wan Dek Haeng Chat) Widely celebrated, especially by governmental agencies, with many activities for children. Observed on this day since 1965; was observed on the first Monday of October from 1955–1964.

National Forest Conservation Day - 14 January วันอนุรักษ์ทรัพยากรป่าไม้ของชาติ (Wan Anurak Sappayakon Pa Mai Khong Chat) Observance promoting the conservation of the nation's forests and the work against deforestation.

Teachers' Day - 16 January วันครู (Wan Khru) Honours teachers countrywide. Schools may observe this day as a holiday, especially those under the authority of the Office of the Basic Education Commission. Observed since 1957.

Elephant Duel Day - 18 January วันยุทธหัตถี (Wan Yuttha Hatthi) Commemorates King Naresuan's victorious elephant duel against the Burmese Uparaja, calculated to be in 1593. Observed on this date since 2007. Officially known as วันสมเด็จพระนเรศวรมหาราช (King Naresuan the Great Day).

Royal Thai Armed Forces Day - 25 January วันกองทัพไทย (Wan Kong Thap Thai) Used to commemorate the elephant duel, but research showed the date to be inaccurate; however, the Royal Thai Armed Forces Day is still observed on this date.

Inventor's Day - 2 February วันนักประดิษฐ์ (Wan Nak Pradit) Commemorates the patent registration of King Bhumibol's Chai Phatthana paddle wheel aerator in 1993. Observed since 1995.

Veterans' Day - 3 February วันทหารผ่านศึก (Wan Thahan Phan Suek) Commemorates the founding of the War Veterans Organization of Thailand. Remembrance ceremonies are held at Victory Monument.

National Artist Day - 24 February วันศิลปินแห่งชาติ (Wan Sinlapin Haeng Chat) National Artists are honoured on this day, which commemorates King Buddha Loetla Nabhalai's birth.

King Nangklao Memorial Day - 31 March วันที่ระลึกพระบาทสมเด็จพระนั่งเกล้าเจ้าอยู่หัวฯ (Wan Thi Raluek Phra Bat Somdet Phra Nangklao Chao Yuhua) Commemorates the birthday of King Nangklao; observed since 1998.

Civil Service Day - 1 April วันข้าราชการพลเรือน (Wan Kha Ratchakan Phonlaruean) Commemorates the enactment of the first Civil Service Act in 1928; observed since 1979.

Thai Heritage Conservation Day - 2 April วันอนุรักษ์มรดกไทย (Wan Anurak Moradok Thai) Commemorates the birthday of Princess Maha Chakri Sirindhorn; observed since 1995.

Consumer Protection Day - 30 April วันคุ้มครองผู้บริโภค (Wan Khumkhrong Phu Borophok) Observed since 1980.

National Labour Day - 1 May วันแรงงานแห่งชาติ (Wan Raeng Ngan Haeng Chat) Observed as a holiday by most of the private sector.

Sunthorn Phu Day - 26 June วันสุนทรภู่ (Wan Sunthon Phu) Commemorates the birth of Sunthorn Phu in 1786.

National Thai Language Day - 29 July วันภาษาไทยแห่งชาติ (Wan Phasa Thai Haeng Chat

National Science Day - 18 August วันวิทยาศาสตร์แห่งชาติ (Wan Witthayasat Haeng Chat) Commemorates King Mongkut's prediction and observation of a total solar eclipse in 1868.

National Youth Day - 20 September วันเยาวชนแห่งชาติ (Wan Yaowachon Haeng Chat) Commemorates the birth dates of Kings Chulalongkorn and Ananda Mahidol.

Mahidol Day - 24 September วันมหิดล (Wan Mahidon) Commemorates the passing of Prince Father Mahidol Adulyadej, the "Father of Thai Modern Medicine".

National Police Day - 13 October วันตำรวจ (Wan Tamruat)

National Nurses' Day - 21 October วันพยาบาลแห่งชาติ (Wan Phayaban Haeng Chat) Commemorates the birth of Princess Srinagarindra the Princess Mother.

Vajiravudh Day - 25 November วันวชิราวุธ (Wan Wachirawut) Commemorates the passing of King Vajiravudh in 1925.

Damrong Rajanubhab Day - 1 December วันดำรงราชานุภาพ (Wan Damrong Rachanuphap) Commemorates the passing of Prince Damrong Rajanubhab. Observed since 2001.

Thai Environment Day - 4 December วันสิ่งแวดล้อมไทย (Wan Singwaetlom Thai) Observed since 1991.

National Sports Day - 15 December วันกีฬาแห่งชาติ (Wan Kila Haeng Chat) Commemorates King Bhumibol's gold medal in sailing at the 1967 Southeast Asian Peninsular Games. Observed since 1986.

King Taksin Memorial Day - 28 December วันสมเด็จพระเจ้าตากสินมหาราช (Wan Somdet Phra Chao Taksin Maharat) Commemorates the accession of King Taksin in 1768.

Main source: wikipedia.org