The Jewish celebration of Passover always happens in the spring, but the first day doesn't always occur on the same date, at least not on our modern Gregorian calendar. This year, it falls on April 19, but here's why it moves around so much from year to year.
Passover is the celebration of the event that led to Moses guiding the Israelites out of Egyptian captivity, a story that's found in both the Hebrew and Christian bibles. Here's a simplified version of the story: The Egyptians had enslaved the Israelites for hundreds of years when Moses went to the Pharaoh of Egypt and told him to let the Israelites go. The Pharaoh refused, so God sent 10 plagues, each one more damaging than the next.
For the final plague, God sent the angel of death in the middle of the night to kill every firstborn son in Egypt. To show that the Israelites were the chosen people, they were given specific instructions to avoid death. They were told to place the blood of a lamb around the doors of their homes so the angel of death would know to "pass over" their homes. When the pharaoh woke to find his firstborn and all the firstborn sons and animals of Egypt dead, he ordered the Israelites out of Egypt.
Festival of Unleavened Bread
In the Bible, Exodus 23:15 says the Festival of Unleavened Bread, which is now called Passover, should be celebrated at the "appointed time in the month of Aviv, for in that month you came out of Egypt." In modern times, the first day of Passover always happens in spring on the 15th of the month of Nissan (formerly known as Aviv) which is the seventh month in the Hebrew calendar. The Jewish calendar is based on lunar months of 29 or 30 days. Each month begins on the new moon. An extra month is added occasionally to help it more closely match the seasons, according to Judaism 101.
In 2019, Passover begins on April 19. In 2020, it will begin on April 8. And, although the date usually falls in April, in 2021, Passover will begin on March 27.