2020 Summer Olympics, originally scheduled to be held in Tokyo, Japan between 24 July and 9 August 2020, were rescheduled for 23 July to 8 August 2021 as a result of the COVID-19 pandemic. The Games of the XXXII Olympiad will keep the name Tokyo 2020 for marketing and branding purposes despite being held in 2021.
At its 1986 Session in Lausanne, the IOC decided to introduce this change. From 1924 to 1992, the Summer and Winter Games were each held in the same year, every four years. This four years period is called the "Olympiad".
The last Summer and Winter Games held in the same year were in Barcelona (Summer) and Albertville (Winter) in 1992. Since then, the Summer and Winter Games are each still held every four years but the Summer Games are celebrated during the first year of an Olympiad and the Winter Games held in the third year.
Source: Bl.ocks.org