If you are not legally married on or before Dec 31 2020, then for each your filing status will be single. Period. Each of you can claim the mortgage interest and property taxes that each of you individually were:
1) Legally obligated to pay, and
2) Actually paid.
If both of your names are on the mortgage and the property deed, then no issues here.
If you are legally married on or before Dec 31 2020, then the "single" filing status is not available to either of you. You must either file as Married Filing Joint, or Married filing Separate.
Since mortgage interest and property taxes are your only deductions, and those are itemized deductions on SCH A, it is very doubtful that your itemized deductions will exceed your standard deduction regardless of weather your married or not, or if you file joint or separate returns if you are married. So unless you have other itemized deductions that will exceed your standard deduction, planning your marriage date for tax purposes is pointless.
If filing single or married filing separate, your standard deduction for 2020 is $12,400
If filing married filing joint, your standard deduction is $24,800.
Again, it is very doubtful your itemized deductions will come anywhere close to your standard deduction, regardless of your marital status, or how you file. Especially with the limits impost on SALT deductions. (State and Local Taxes).
