One of the most important rights US citizens have is the right to vote. Voting is essential to maintaining a well-functioning democracy.

To highlight the gravity of that fundamental right, Google on Tuesday dedicated its Doodle to National Voter Registration Day.

Celebrated each September since 2012 across the US, the nonpartisan civic holiday is part of a coordinated effort to get eligible citizens registered to vote.

Volunteers and organizations across the US will hit the streets Tuesday to raise awareness of voter registration opportunities to eligible citizens who may not register otherwise.

More than one in five eligible Americans aren't registered to vote, according to US Census Bureau data.

More than one in five eligible Americans aren't registered to vote, according to US Census Bureau data.

But over the years, the holiday has helped nearly 4.7 million people register to vote, according to the National Voter Registration Day organization.

"Every year, millions of Americans find themselves unable to vote because they miss a registration deadline, don't update their registration, or aren't sure how to register," the organization says on its website.

"National Voter Registration Day wants to make sure everyone has the opportunity to vote."

"National Voter Registration Day wants to make sure everyone has the opportunity to vote."