The Mishnah Berurah (90:28) quotes a teshuvos Radvaz, who discusses a prisoner that was granted permission to daven one day with a minyan. The question was which day to choose? Perhaps he should request to go to shul for Yom Kippur? Perhaps he should ask to go to shul on Purim? The Radvaz advises that he should go to shul at the first opportunity he can, and he shouldn’t push it off for a later date. Perhaps he will lose his chance to daven with a minyan.
Notice that the two days the Radvaz ponders about were Purim and Yom Kippur. This tells us about the importance of these two holy days. He didn’t even mention Rosh Hashanah. Purim and Yom Kippur are at the top of the list. ~ R’ Elimelech Biderman Shlita