The Lithuanian calendar is unusual among Western countries in that neither the names of the months nor the names of the weekdays are derived from Greek or Norse mythology. They were formalized after Lithuania regained independence in 1918, based on historic names, and celebrate natural phenomena; three months are named for birds, two for trees, and the remainder for seasonal activities and features. The days of the week are simply ordinal numbers. The Lithuanian calendar shows some similarities with the Slavic calendars.