Sunrise to sunset on April 18 this year is the Fast of the Firstborn.
This fast commemorates the saving of the Israelite firstborns from the final plague: death of the firstborn. All the firstborns of Egypt, the children of the garbage man to the baker to Pharaoh himself, died.
Only certain firstborns are required to fast. There are three requirements: (1) A male (2) firstborn (3) over bar mitzvah age. If the male firstborn is under bar mitzvah age, the father generally fasts on his behalf. If you are the first son but have an older sister, you are not a qualifying firstborn. There are differences of opinion when the person is (a) the firstborn of only the father (aka, the mother had a child previously), (b) a convert, or (c) born through cesarean section (C-section). If you are the second son when the first son passed away during the first 30 days of life, you are obligated to fast.
Just because you're obligated to fast doesn't mean that you will actually fast all day. It is common practice to break the Fast of the Firstborn at a festive seudah (meal) after the morning service (shacharis). If there is a bris (circumcision) or redemption of a firstborn, that seudah can count. Typically, the synagogue will host a siyum, which is a festive meal celebrating the completion of a tractate of Talmud. You need to hear the completion of the tractate and understand it. I'm not sure what qualifies as understanding it, so that is a question for your rabbi.
Of course, if you are otherwise exempt from fasting because of health reasons, you are not obligated to fast.