The United States flag started out with 13 stars, and 13 stripes, for the 13 original states. The flag slowly accumulated stars as new states were added to the Union. In 1959 it reached its present form of 50 stars, with the addition of Hawai’i. For obvious patriotic reasons, it was decided in 1818 that
after a new state had been confirmed by Congress, that state’s star would be officially added to the flag on the next 4th of July.

This led to a fun historical oddity: even though Wyoming and Idaho became states within a week of each other in 1890, their stars were put on a year apart. How did this happen? Well, Idaho became a state on July 3rd, 1890 and Wyoming became a state on July 10th, 1890. Because Wyoming just missed the July 4th deadline, its star was added 1 year later, in 1891.