It became a mass phenomenon only during the Edo period, when the samurai warrior class and the common people started to celebrate it too. Since then, the tradition preserved until today and now many people are wearing for hanami a traditional attire like kimono or yukata.
EXIF info:
Nikon D700
Lens: 24-70mm F/2.8G
Focal Length: 62mm
Aperture: F/5.6
Shutter Speed: 1/400s
ISO Sensitivity: ISO 320