Although, technically, the landing on Playa Giron took place on April 17th, air attacks on Cuba started on April 15th as a prelude to the landings. As we all know the invasion failed, some have blamed the fact that the air strikes were suspended by President Kennedy, others have blamed intelligence leaks that let Fidel know about the invasion in advance. Both of those are likely part of the story, the air strikes were suspended before they had done any real damage to the Cuban military, and on the days preceding the invasion the Cuban government rounded up tens of thousands of those it considered potentially opposed to the revolution.
Whatever the cause of the failure, the one thing the Playa Giron invasion did was solidify Fidel’s position in the island. From that point forward no dissent was tolerated, opposition groups were rounded up and its members imprisoned or executed. Opposition media was shut down or taken over by the government and anyone not seen as actively loyal to the revolution was deemed a counter-revolutionary. The invasion also gave Fidel an obvious example that he could point to of the US government’s desire to overthrow him, the level of nationalist support that got him was enormous.
The invasion also marked the date when Fidel declared the revolution to be communist. Up until that point there had been an obvious entanglement with the USSR, but not necessarily with the USSR political model. After the Playa Giron invasion Cuba was, for all intents and purposes turned into a soviet state.