(n): the date on which an event occurred in some previous year (or the celebration of it)
(n): a span of 1000 years
(n): an amount of time
(n): a sum or aggregate of one thousand (especially one thousand years)
(n): the cardinal number that is the product of 10 and 100
(adj): of or relating to the doctrine of the millennium
(adj): relating to or consisting of 1000
A millenary word (adj., relating to thousands) seems apt for my thousandth addition. If you'd like to celebrate, please send millinery by the millenary (n., a sum of one thousand).