The partnership’s tax year ends on the date of termination which is the date the partnership winds up its affairs.
See https://www.irs.gov/instructions/i1065#idm140672607553008
For future reference:
Generally, a domestic partnership must file Form 1065 by the 15th day of the 3rd month following the date its tax year ended....
See https://www.irs.gov/instructions/i1065#idm140672607526672
The 2019 Form 1065 may also be used if:
