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.