If you want to run your Membership dues on the same day each month you will need to have new members who join mid month be able to pay a prorated fee for the first month.
ADD their new membership type with the STARTING DATE
Check the Create Billing Cycle between START DATE and NEXT PAYMENT DATE
This will let the new membership type be billed on the NEXT billing cycle (ex. 2/01), and it will charge them the prorated price for January membership as well the full membership price for February (2/01).