Hi,I'm Jules, a LPC,I am reviewing your question now, and will post back with your thorough reply briefly :)
It is not imperative to invite spouses or significant others or "plus ones" to a holiday party, but all employees (single or married) regardless of relationship status, should be invited. If you are in charge of the party, clearly convey to employees who is invited to the party. If spouses or children are not included, say so. Another thing to consider is if the party is a daytime or evening party more convenient for attendees? For employees with children, arranging child care for an evening event may be an issue. If you plan a party during office hours, however, make sure everyone can attend. I have been to both types of holidays parties, where spouses, significant others, and children are included, but I have also attended exclusive parties that are employee only, so it is not necessary to include partners, but just convey so in your invitation :)
This article is helpful across the board.
Please let me know if this is useful information. I am happy to send more if necessary. If you have been satisfied with the information given, please provide a positive rating (3 stars or better) as it is the only way that experts receive credit for the answers given. Even after a positive rating, it does not mean that the conversation has to cease, communication can continue as related to original question. I hope to hear back from you soon :) Thank you! Jules