The most efficient way to return the last day of any month.
Let’s assume A1 has a date in it, and you want the last day of that month:
=DATE(YEAR(A1);MONTH(A1)+1;0)
There is a slightly shorter method but it can only be used when/if the Analysis Toolpak is installed. That is, Tools>Add-ins and check Analysis Toolpak. Then you you can use the EOMONTH function as shown below
=EOMONTH(A1;0)