Get last day of the month in Ruby

I made new object Date.new with args (year, month). After create ruby added 01 number of day to this object by default. Is there any way to add not first day, but last day of month that i passed as arg(e.g. 28 if it will be 02month or 31 if it will be 01month) ?

use Date.civil

With Date.civil(y, m, d) or its alias .new(y, m, d), you can create a new Date object. The values for day (d) and month (m) can be negative in which case they count backwards from the end of the year and the end of the month respectively.

=> Date.civil(2010, 02, -1)
=> Sun, 28 Feb 2010
>> Date.civil(2010, -1, -5)
=> Mon, 27 Dec 2010

To get the end of the month you can also use ActiveSupport's helper end_of_month.

# Require extensions explicitly if you are not in a Rails environment
require 'active_support/core_ext' 

p Time.now.utc.end_of_month # => 2013-01-31 23:59:59 UTC
p Date.today.end_of_month   # => Thu, 31 Jan 2013

You can find out more on end_of_month in the Rails API Docs.