Lunar, Solar and Lunisolar
The biblical year is lunisolar; the Julian and Gregorian calendars are purely solar. That mismatch is why Pascha must be computed rather than simply looked up.
Three families
A lunar calendar tracks moon cycles and drifts through the solar year. A solar calendar tracks the sun and ignores the moon. A lunisolar calendar tracks both, inserting an extra month periodically to keep the lunar months aligned with the agricultural year — the Hebrew calendar's approach.
The nineteen-year cycle
235 lunar months come very close to 19 solar years — the Metonic cycle. Both the Hebrew intercalation scheme and the Christian Paschal tables lean on it. The approximation is good but not exact, and its small error is a second, separate source of drift in the Paschal computation, distinct from the leap-year error.
Consequence for Pascha
Because Pascha depends on a full moon and an equinox, the Church needed a computation — the computus — rather than a fixed date. Whole centuries of ecclesiastical mathematics exist because of this one requirement.