Set the Sales Effective Date to Today's Date using Workflow

To set the Sales Effective Date to today's date, see below steps 
 
1. Navigate to  Customization > Scripting >  Workflows > New
2. Record Type: Transaction
3. Subtype: Sales Order.
4. Execute as Admin is checked.
5. Release Status: Released.
6. Event Definition: On Create.
7. Trigger Type: Before Record Load.
8. Click New State > Save.
9. Click State 1 > New Action.
10. Set Field Value.
11. Trigger On: Entry.
12. Parameters Section > Set Field Value = Sales Effective Date. 
13. Date = today. 
14. Click Save.