ホワイトデー is “White day” in katakana. This is like reverse Valentine’s day in Japan; guys return the favor on March 14th to the girls for the chocolates they received on February 14th. This means they give girls chocolates or small presents, like handkerchiefs or hand towels, keychains, etc.
The stores will usually just change the sign from Valentine’s day to White day on the chocolates displays. Departments stores and shops will put out displays of small presents mentioned above, usually with cute designs or packaging. It is not as big as Valentine’s day, but there are plenty of displays to remind the guys to buy the girls a small present or chocolate.