When end user created a new sales order with VA01, default First Date wasn't today, why? 

Note: 
1. Before today, default First Date was always today. 
2. Nobody change system configuration. 

Although you mention that nobody change the system configuration, it is very unlikely that the system will mis-behaved after one day.

Usually, after checking, you will find that someone have actually change the configuration as it could not be a software bug since you have been using it for quite sometime without any problems.

The date is control by each Sales Order Type for each Sales Document type whether is it a 
- OR - Standard Order, 
- RE - Returns etc.

Verify the Sales order type configuration with the following path:

IMG: Sales and Distribution --> Sales --> Sales Docs --> Sales Doc Hdr --> Define Sales Doc Types (transaction vov8) will let you control this by sales document type. 

There is one field (Lead time in days) which "specify the number of days after the current date that the proposal for the requested delivery date in the sales document should be". 

This should be blank if you want the system to propose current day for delivery date. 

Configure the system to auto proposed the Sales Order date
Auto proposed all the dates when creating Sales Order

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