Date::MONTHNAMES

月の名前、日の名前。

require 'date'

p Date::MONTHNAMES
p Date::DAYNAMES
p Date::ABBR_MONTHNAMES
p Date::ABBR_DAYNAMES

実行結果です。

[nil, "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"]
["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
[nil, "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"]

月の名前の配列が実質1オリジンになっているのに注目。なるほどね。
追記：ぜんぜんRubyとは関係ないのですが、「月の名前」という語を書くと、わたしの頭の中で「肉の名前」という語が出てくるのはなぜでしょう。西尾維新の読み過ぎでしょうか。