pikku- (“little”) + lauantai (“Saturday”), probably a calque of Swedish lillördag. Possibly from a custom of organizing "weekend activities" on Wednesday...
onsdag, torsdag, fredag, lördag, söndag (Category: sv:Days of the week) lillördag onsdag in Svensk ordbok (SO) onsdag in Svenska Akademiens ordlista (SAOL)...
lillördag (more common) Compound of lille (“little”) + lördag (“Saturday”). From the practice of giving servants a free day in the middle of the week...