Namazake is completely unpasteurised, unlike normal sake (pasteurised twice), nama-chozō (生貯蔵, stored unpasteurised but pasteurised at bottling) and nama-zume (生詰, pasteurised before storage but not at bottling).
[I had no idea this existed, but no joke, there is a company that manages “XX Day” in Japan… apparently there’s an “Amazake Nouveau” day that changes yearly. “Tasty Butter Day” is 21 August. “Tokyo Banana Day” is 7 August. “Baby Shower Day” is 6 June? How does that work? Do women have to coordinate giving birth?? Anyway. Clearly taking money to give some attempted legitimacy to companies making things up.]
25 June is when the summer sun starts to really heat up, and demand for refrigeration increases, so Gekkeikan chose it as the perfect moment to enjoy some chilled namazake. It also commemorates their industry-first use of super-fine filtration technology to launch a namazake that could be transported without cooling on 25 June 1984. This allowed the taste of freshly pressed sake, previously only available at the brewery, to be enjoyed by drinkers at home. (Cans are also a good solution for keeping namazake away from damaging light and oxygen: You can with a can.)