Disclaimer: I have never worked as a CNA - I am merely speaking from what I experienced in nursing school/as an RN. As a general rule: Acute care setting would offer a full time position with 12-hour shifts 3x a week. Long-term care would offer 8 hour shifts 5x a week. As far as working only weekends Friday/Saturday/Sunday, that would be up to the facility. The nature of 12-hour shifts usually comes with alternating days throughout the week (i.e. a different 3 days every week). There were students in my programs that were CNA's/LPN's that worked nights or had relationships with their employers
that allowed them to choose the days they worked to match our program requirements. In the latter situation, these were long standing employment situations, no one walked into a job with that kind of flexibility. As an alternative, there are also CNA "float pools" at some hospitals and if you were lucky enough to gain employment at more than one facility, you may get three shifts that way. More likely, since you are wanting to work the (less desirable to most) weekends. But there's no "guarantee" here - so I mention this more as a "last resort"
Please know whatever route you take. Working full-time - especially nights - requires a level of commitment I hope you are prepared for. Please seek advice from someone you trust on this. I hope you also consider the wages you will be making as a CNA and you and your fiance are able to budget accordingly.