Get dynamic month(column name) which needs to be updated

I have a query which gives me some data. I have 12 months say. but it will update the data for the column say month12 (because current month is december)

there are 12 columns based on each month. here is my query,

In my select query I need to find which month's columns needs to be updated.

Below is my query

select  --a.emp_mkey,
                                @Total_day =  Sum(total_day), 
                                @Days_worked = Sum(days_worked)       
                                from emp_mon_day a      
                                where a.emp_mkey =  @emp_card_no
                                and a.month = @actualMonth and Year = @actualYear 
                            group by emp_mkey       

                            if(@Days_worked > 0)
                             BEGIN
                                set @actualleavedays =((1.75) / @Total_day) * (@Days_worked)
                             END 
                        print @actualleavedays

I am using SQL - server 2005

UPDATE

so what month will come here -- month1, month2, ...... ???

-- update p_leave_allocation_14122015 a
                    --      set a.month1 = 8.18
                    -- where year = a.actualYear and a.emp_card_no = @emp_card_no

Do let me know if you want to view whole procedure.