In "October" what does it mean that October is "the treasurer of the year"?

A "treasurer" is usually the person in a group or society who is responsible for the accounts and wealth—all earnings are paid to them, and it is up to them to determine what to do with what has been accrued. October, in Dunbar's poem, is the "treasurer of the year" in that "all the months pay bounty to her store," filling her "brimming coffers" with the literal fruits of their labor. October, however, is not a diligent treasurer, in Dunbar's judgment. Rather than guarding the bounty that has been given to her, she "spends all her wealth in gaudy dress."
The "bounty" paid to October is, of course, the harvest; many fruits and vegetables are ready to be collected at this time of year, the culmination of months of hard work on the part of both the plants and their human cultivators. Dunbar characterizes October as a "lavish" young girl, taking all the bounty given to her and choosing to exhibit it in a blaze of glorious color (a clear nod to the fall colors and turning of trees.) October's habits as treasurer seem to be born of the knowledge that "Frost stalks o'er her way" no matter what she does, and so she may as well "live...joyously."