ONLY THE SANHEDRIN CAN SANCTIFY THE NEW MOON OR ESTABLISH LEAP YEARS

In Mishneh Torah Hilchot Kiddush HaChodesh (laws for sanctifying the new moon) 5:1 Moses Maimonides (Rambam) explains that only the Sanhedrin has the authority to sanctify the new moon or establish a leap year to reconcile the lunar calendar with the solar calendar.