According to regulations, there has to be a minimum of 30 days gap between the 1st and 2nd SIP installment. After the 2nd installment, it will be deducted and invested monthly.

For example: If your SIP date is the 1st of every month.

1st installment: 10 March 
2nd installment: 1 May 
3rd installment: 1 June

...and so on.