Unfortunately, the modern Jewish calendar no longer calculates the festival dates according to the scriptures. In order to produce calendars years in advance, they have chosen to estimate the first day of the month rather than use the sighted new moons. (Psalm 104:19 He made the moon to mark the festivals)
In addition, a number of non-biblical traditions have been added to the calculation of the festivals. For example, the one-day celebration given in scripture has been changed to a two-day celebration by Judaism. This causes the Jewish calendar to show many festivals beginning 24-48 hours before their scriptural dates.
Here you will find the festival dates calculated according to the sighted new moon, which follows the biblical and ancient process.