(note: I am not talking about any of the recent issues with the fishing industry)
Slavery in Thailand was officially abolished on April 1st, 1905. There's actually a picture of Rama V freeing the slaves on the back of the 100 baht bill. Every source I've been able to find, in Thai and English, corroborates this.
Except I read the law and it doesn't abolish slavery at all. It starts with some platitudes about how Thai slavery isn't really that bad and says, "We now deem it time to take more sweeping measures which will gradually result in the entire disappearance of slavery from Siam." Then it goes on to decree no one can be born or made a slave from now on and exempts some districts. It also says that owners of debt slaves need to gradually reduce the amount owed, but makes no mention of the other six categories of slaves. You can find the full translation here (ctrl+f "abolition of slavery").
I'm used to the bizarre alternate reality that is Thai historiography but this issue has proven stubbornly opaque. Every time I have brought this up I get the same non-answer: "Slavery in Thailand was different from in the West."
So my question is this: when did legal slavery end in Thailand? Was there some later law that did away with it completely, or were there people born in 1904 who were legally bonded until they died in the 70s or 80s?
edit: I got the year wrong in the title, should be 1905.
I apologize if this is an inappropriate top level comment, but would it be possible to hear some examples of what you mean by the bizarre alternate reality that is Thai historiography? I'm not an expert in Thai history at all, but studied Korean history at a Korean University and had a similar experience with historiography of Korean-related history.
"§ 3. All children born of parents who are slaves shall be free without the execution of the condition stated in the law of Pe& Chau.
§ 4. No person now free can be made a slave. If any person now a slave shall hereafter become free, he cannot thereafter again become a slave. "
You can't be born into slavery and once you are free, you can't become a slave again.
Once everyone (in 1905) either became free or died, slavery would have been abolished by virtue of the two above-quoted provisions.
Finally I can chime in with (somewhat) useful information!
Sounds to me that this 1905 law was the first step into a gradual abolition of slavery, that was then quickly accomplished without the need for new laws (fishing industry and who knows what else notwithstanding).
As a comparison, Chile (where I live) had gone through a gradual abolition of slavery as well, although earlier and much faster (aided by the fact that there weren't many slaves in the country to begin with). In 1811, the "libertad de vientres" bando (edict) established that no new slaves could be imported into the country and the children of slaves born from that point on would be free men; afterwards, in 1823, the abolition law declared all slaves in the country to be free and also those from abroad who would set foot on it.
I now wonder if, on the paper at least, slavery wasn't properly ended in Thailand. :-/
Even according to the standard narrative taught in schools, the abolishment of slavery in Thailand never happened overnight. In fact, it is often emphasised that the approach King Chulalongkorn used, a gradual phasing-out of the practice, helped avoid direct confrontation with certain elite groups and thus assured the policy's success.
This gradual abolition was first introduced in 1874, with a royal act stipulating that those born into slavery since 1868 be free upon reaching twenty-one years of age. Other laws enacted in 1884, 1890, 1897 and 1900 further clarified or expanded these regulations. The 1905 law, while often regarded as abolishing slavery, didn't immediately outlaw the practice. Older slaves remained, though with gradually reducing caps on their prices.
I'm not sure of the specifics, but it appears that at this point, only two categories of slaves were recognised, rather than the traditional seven: war captives and debt slaves. In Breaking the Chains: Slavery, Bondage, and Emancipation in Modern Africa and Asia (1993), Martin A. Klein quotes John Bowring's observations that slave prices varied from 80 to 160 baht for males and 60 to 100 for females. This means they would have been freed within a few years after the four-baht-per-month reduction was implemented. War captives also appear to have had fixed prices stipulated for them: 25 baht for males and 32 baht for females, according to the 1900 law concerning the northwest region. There was also an age cap of sixty.
Further laws that followed the 1905 act would finally criminalise slavery. These include the 1908 penal code (translation), section 269 of which prohibited the sale and acquisition of slaves (exempting those which were legal under the 1905 act). Acts from 1911–13 expanded the coverage of previous laws. Klein notes that slavery legally ceased in 1915, though the significance of the date isn't actually explained.
The commemoration of a specific date as "abolition of slavery day", as far as I can determine, appears to be a fairly recent thing, and has no official status. There is no national observance for the event. Also, differing dates appear to be used. In addition to 1 April mentioned by the OP, some have instead marked 21 August (the date of the 1874 act) as abolition-of-slavery day. So my short answer would be: as the abolition of slavery was a gradual process, it was never marked by a clear-cut date. The question's premise of slavery being "officially" abolished on April 1st, 1905 appears to be faulty.