In general, Jewish Holidays begin the evening before the date specified. This is because the Jewish day actually begins at sundown on the previous night. Sometimes, for clarity, the Erev holiday is also included to indicate that the holiday begins the evening before.
For example, in the April 2015 calendar below, Erev Pesach is listed as April 3rd and the first day of Pesach is listed as April 4th. This means that the holiday of Pesach begins on the evening on April 3rd.
And, Rosh Chodesh Iyyar is listed on April 19. This means that Rosh Chodesh begins on the evening of April 18, even though the Erev is not explicitly mentioned on the calendar.