• Tom will bring the book tomorrow.
(There is one specific book that Tom will bring tomorrow.)

• He will arrive on the first Tuesday in July.
(There is only one first Tuesday in July.)

• He sailed on the Pacific Ocean.
(There is only one Pacific Ocean.)

The indefinite article is used when the noun could be one of several different nouns.

• Tom will bring a book tomorrow.
(Tom will bring any one book.)

• He will arrive on a Tuesday in July.
(He will arrive on one of four Tuesdays in July.)

• He sailed on an ocean.
(He sailed on any one of the world's oceans.)