I've searched and can't find anything that similar to what I need.

In my layout I have a DATE IN, a DATE OUT and a RECORD COMPLETE field.

When the record is created, the DATE IN field is auto-entered w/the current date and RECORD COMPLETE is set to NO.

When the record is completed the users fills in the DATE OUT. When a date is put in the DATE OUT field it triggers an auto-enter calc that changes RECORD COMPLETE to YES.

I also have a CATEGORY field specified as a checkbox set. I need the CATEGORY field to be validated only when the RECORD COMPLETE = YES.

It should not allow the CATEGORY field to be empty after the record is complete. It does not need to validate if the record is not complete.

Also, there may be instances where the CATEGORY field is never entered, however, it still needs to have a value selected.

I need it to show a custom message when validation fails.

Try a validation of

Record Complete = "no" or not IsEmpty(Category)