Date::MONTHNAMES.each_with_index.collect{|m, i| [m, i]}
=> [[nil, 0],
["January", 1],
["February", 2],
["March", 3],
["April", 4],
["May", 5],
["June", 6],
["July", 7],
["August", 8],
["September", 9],
["October", 10],
["November", 11],
["December", 12]]
Сокращенный выбор с опцией по умолчанию
Date::ABBR_MONTHNAMES.compact.each_with_index.collect{|m, i| [m, i+1]}
.insert(0, ['Please Select', nil])
=> [["Please Select", nil],
["Jan", 1],
["Feb", 2],
["Mar", 3],
["Apr", 4],
["May", 5],
["Jun", 6],
["Jul", 7],
["Aug", 8],
["Sep", 9],
["Oct", 10],
["Nov", 11],
["Dec", 12]]