How can I create a trigger in Access 2003

I need to move data in a table from one field to another.

Table                           Field1      Field2      Field3       Field4        Field5        Field6
BudgetActual Detail    Budget1  Actual1   Budget2  Actual2    Budget3    Actual3
                                    1000            0           0              0               0               0

                                             Period 1                Period 2                     Period 3

If we did not get any Actual1 in Period 1, the 1000 will be move to Budget2 which is in Period2

This will be a Trigger that will run every "First" and "Sixteenth" of the month.

Thank you.