The date of the Passover holiday changes each year on a standard calendar, even though it is always the same on the Jewish calendar. This can sometimes make it confusing when trying to figure out the date of the first seder!
In 2017, dates are as follows:
Passover begins at sunset on Monday, April 10, 2017.
Wondering when is the first seder in 2017? The first seder for 2017 will be on Monday night, April 10, 2017.
The second seder in 2017 is Tuesday night, April 11, 2017.
Passover will end after sunset on Tuesday, April 18, 2017.