Attosecond Day in Japan, observed on October 18th, is a day officially recognized as a commemorative day by the Japan Anniversary Association. Registered in 2025, this year marks the inaugural observance of the event.

Attosecond Day in Japan highlights the field of attosecond science, in which researchers in Japan have been intensively investigating physical and chemical phenomena occurring on the timescale of hundreds to tens of one quintillionth of a second (10⁻¹⁸ seconds) in cooperation with researchers around the world.

Organized by the Institute for Attosecond Laser Facility at the University of Tokyo, the event will feature academic lectures and talk sessions on attosecond and ultrafast science by researchers working at the forefront of the research field, along with related demonstrations and exhibitions. Special emphasis will be placed on pioneering research results in attosecond and ultrafast science achieved by researchers in Japan in the past decades. This first edition aims to provide an informative environment for students, researchers, and the public to engage with recent developments in this emerging area of scientific research.