Ok, I've done this before, depending on what you think will make you feel better, either volunteer somewhere or make it a total pamper yourself day. And it's not better to volunteer just because it sounds more ultruistic. If it will make you sadder and feel more alone, then don't do that. Only do it if you think it will really uplift you, because charity done without joy isn't really giving much. It's perfectly ok to take a day and celebrate yourself, especially if that will make you happy, because the happiness will spill over to others anyway and better when you feel good. If you like to cook, cook your absolutely favorite comfort food. If you don't, then buy your favorite comfort food (in moderation), get a book you really want to read, watch the movie, do an at home spa day, whatever pampering is to you.
But then, going forward, try to avoid this in the future. A holiday alone every so often happens, but if it is a regular occurence, then start trying to connect with others who are similarly situated and create an extended family of friends who need you as much as you need them. Creating Extended Families is possible and necessary for the long term. Wishing you happiness. - Holi