I can't read Spanish, but skimming the tables in that article, it seems a lot of switches in the 1910s–1930s were legally at 25:00.  So presumably a 25:00 Saturday switch meant that it was legally considered Saturday right up to and including what we would call "00:59:59 Sunday", and only became legally Sunday once the clock fell back an hour to 00:00:00.

I think that level of detail is outside the scope of the data itself (and I'm pretty sure we wouldn't have a great way to represent it anyway), but is certainly fascinating enough to include in our commentary, if I interpreted it correctly.

Can someone with a better comprehension of Spanish confirm and/or translate the relevant parts?