The calendar of Jubilees is not decoration. Its year has 364 days, in four quarters of thirteen weeks, each quarter of three months of thirty, thirty and thirty-one days. The arithmetic is the point: 364 divides by seven exactly, so every date falls on a fixed weekday, year after year. No festival can drift onto a Sabbath and force a compromise between two obligations.
Against this the book attacks lunar reckoning directly, warning that those who observe the moon will disturb the seasons and eat unclean things and keep the feasts on the wrong days. That is a charge aimed at the temple in Jerusalem, whose calendar was lunar-solar with months set by observation of the new moon and intercalation to hold the year to the seasons.
The consequence is a permanent schism in practice rather than a theoretical disagreement. A community keeping the 364-day year celebrates Passover and the Day of Atonement on days when the temple is doing something else, and the temple's Day of Atonement falls on an ordinary day for them. Shared worship becomes impossible without one side conceding. The same 364-day calendar appears in the Astronomical Book of 1 Enoch and in calendrical texts from Qumran, which is one reason the Qumran community's sectarian separation is usually read as calendrical as well as priestly and political.
Jubilees never explains how a 364-day year is kept in step with an actual solar year of about 365 and a quarter days. No intercalation scheme is given, and whether its holders used one, and what it was, remains unresolved. Some scholars propose a periodic added week; the text itself is silent, and the silence is genuine rather than a gap in the manuscripts.