This is the date from which the SIP is deducted from your account each month.
The SIP date is chosen by the investor when the SIP is set up. If there is a holiday on the SIP day, the SIP can be deducted 1-2 days before or after the scheduled SIP date.
This is why it is recommended that investors set up their SIP on a date a few days after they receive their income in order to be secure in the event that the income is delayed in a given month.
Multiple SIPs may be set up in the same mutual fund, resulting in multiple SIP dates in the same mutual fund.