YANBU at all. This sort of surprise is just AWFUL. The person / people planning the surprise are being totally indulgent. THEY get to enjoy the delicious anticipation, THEY have weeks of looking forward to seeing the surprised faces, THEY are massaging their own egos with the fabulousness of arriving unannounced, just so they can see 'delight' on the face of someone who was denied all those weeks of anticipation themselves.
When my ex's stepfather was terminally ill, ex's brother decided he wanted to visit (from his new life abroad) and turn up, unannounced, at DMIL's door, thinking how she would be so thrilled to see him. Thinking how much HE was looking forward to seeing her delight.
It took a lot of persuasion for him to see how much he pleasure HE was keeping to himself, how much HE was enjoying the planning, how much HE was grinning to himself with delight that he had masterminded this fantastic plan, to see his mother's face light up.
Eventually, he agreed she could know that he was going to come and visit, a few weeks before his flight. It was in the days before Skype and Facetime, he couldn't see her face, but we did. She was beside herself with joy. Her husband was desperately ill, she was still working part time, she had a lot of wonderful, supportive friends. The delicious anticipation of her younger son's forthcoming visit gave her something else to focus on. She could plan who they'd lunch with, who they'd visit, where they'd go, what they'd do, what she would cook for him, what they would shop for. She could talk (and talk, and talk!) to her friends, to all her family, about her thrill that he was coming over for a couple of weeks... she could wallow in the thrill of anticipation, enjoy looking forward to seeing him. It was fantastic. Had he insisted on his original plan to surprise her by turning up at the door unannounced, he would have had those weeks of looking forward to it - and so would we - but dear MIL, for whose benefit it was planned, would have missed weeks of pleasure anticipating his coming.
Far too many surprises are planned for utterly selfish reasons, though the planner thinks it's all about the recipient, it really isn't - it's about the planner's delight in knowing their delicious secret. Don't do it. Share the pleasure, don't keep it a secret. Let your DP look forward to it, lengthen the joy of their Christmas by letting them plan for your visit. Do you really think you'll enjoy it less if you don't let them in on it?