Use of Case Statement in Ruby

def monthname(month)
 case month
  when "jan"
    full_month="January"
  when "feb"
    full_month="February"
  when "mar"
    full_month="March"
  when "apr"
    full_month="April"
  when "may"
    full_month="May"
  when "jun"
    full_month="June"
  when "jul"
    full_month="July"
  when "aug"
    full_month="August"
  when "sep"
    full_month="September"
  when "oct"
    full_month="October"
  when "nov"
    full_month="November"
  when "dec"
    full_month="December"
  else
    puts "Invalid abbreviation"
  end
 return full_month
end

puts "Enter the month abbreviation"
a=gets.chomp()
puts monthname(a)