Choose a date close to your salary credit/cash inflow.
Invest first and allocate money for essential expenses, so that you will be left with little money for discretionary expenses.
Missing continuously 3 SIPs leads to termination of SIP investment by the fund house & the bank may penalize for dishonouring the payment. So, make sure there is sufficient balance.
Either one date (close to the beginning of the month) can be set or splitting among two dates makes sense & hassle- free. Depending upon the risk tolerance & investment objective (goal), selecting an appropriate mutual fund scheme & continuing the SIP is the paradigm.