I think it sad that you would consider such a wish to be "dumb".
It's dumb because why would I wish someone Chag Sameach on Shemini Atzeret when they 1)have no clue what it is, 2)if they did it would not mean anything to them as they do not celebrate it, and 3) If they are like many on FR would instead take the opportunity to either 'witness' to me or tell me about their lovely Replacement Theology?
How sad.
How sad it is that your friends who profess Christ seem to have a rather singular lack of sensitivity to your feelings.
If I were your friend, and you were to wish me a Merry Christmas, and I knew that your December 25th meant work or Chinese food or a movie for you, I might be inclined to respond to your wish to me for a Merry Christmas with something like this:
Thank you, my friend. And my best wishes to you and all those close and dear to you for a most joyous Christmas Day. On that day, whether you are at work -- or doing whatever you are doing -- please know that you are in my thoughts. And also please know that I will be giving thanks on that day that you are included as a part of my world. Best and most sincere wishes for a most Merry Christmas."
It is precisely because I would understand that Christmas Day for you is not a "merry" day (because of the circumstances of work or food or movies) that I would want to wish you joy.
How sad it is that your Christian friends feel -- for some reason -- that they are u nable to take the simple act of kindenss (wishing you a Merry Christmas) when they know that you will not be having good day.
Excuse me if I presume too much here, but it does rather seem to me that you appear not to wish to share with others outside your own faith community the happiness that comes to you from celebrating Shemini Atzeret because some (many?) have responded to expressions of your own religious joy by witnessing to you.
It is almost as that those who witness to you feel that it is their job to drag you into the Kingdom of God.
I won't bore you with my own view on any of that, except to say that my understanding of the Law is that the Greatest Commandment is to Love God, and the Second Greatest Commandment is like it -- Love Your Neighbor as Yourself.
My God commands me to love you first above anything else. Witnessing is not the greatest commandment. It is also my belief that a sovereign God is quite capable of getting the people He wants into His kingdom.
I love.
God is the one who converts peoples' hearts.