How to create a DateTime object that represents the last day of a given month?

You can use the FluentDateTime package. It provides the LastDayOfMonth() extension method plus a number of other extremely useful extensions.

You can install it through nuget

  

PM > install-package FluentDateTime

Use

using FluentDateTime;
...
DateTime ultimoDiaDoMes = qualquerData.LastDayOfMonth();

And you can also do it in a way (which I think) simpler than this in your answer

var qualquerData = new DateTime(data.Year, data.Month, 1);
DateTime ultimoDiaDoMes = qualquerData.AddMonths(1).AddDays(-1);

I found this answer in SOEn

To get the day only, the DaysInMonth method that receives as parameters the month and the year and will return a int which will be the last day.

With the last day retrieved, I created a new object of type DateTime using the month and year of my original object plus the day retrieved.

var data = new DateTime(2015, 11, 7);
var ultimoDia = DateTime.DaysInMonth(data.Year, data.Month);
var dataUltimoDia = new DateTime(data.Year, data.Month, ultimoDia);

  

How do I find out the last day of this month?

var ultimoDia = DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month);