A birthday where the guests solve a murder is memorable in a way a restaurant booking is not. The trick is to plan it around the case without letting the case swallow the birthday.
The simplest way to keep the focus on the guest of honor is to give them the case. They open the room, they read the reopening memo aloud, and they cast the final vote if the group is split. They should not read anything in advance; the pleasure is in discovering the file with everyone else. If they are a known puzzle lover, give them the hardest folder, usually forensics, and let them show off.
Send the invitation as a case summary rather than a party flyer. Something like: "File reopened. A tavern owner, found in his own walk-in cooler in 1993. Six suspects. One evening to close it. Saturday, 7 pm." It sets the tone without asking anyone to dress up. Tell guests it takes a few hours so nobody plans to leave after an hour. Mention that the case involves a fictional death so nobody is surprised, and that it suits adults and older teens.
For a birthday, you want a case with a strong opening and a satisfying reveal. The Harvest Ball is set at a party itself, a grain baron who drinks and dies at the head table, which gives the evening a nice mirror. Last Call at the Bluebird is free, so you can test it with the birthday person's closest friend a week earlier and make sure the group will enjoy the format.
Do not try to cut the cake during the vote. A rhythm that works: arrive and eat, then an hour of reading, then cake and presents as a natural break while everyone's theories settle, then the final hour of argument and the reveal. The break matters more than you think. People come back from it with fresh eyes, and someone usually says "wait, what time did that witness say she left?" within five minutes.
If you have more than eight guests, split them into teams that race each other on the same case. Each team opens its own online room so every member reads on their own phone. Give the teams names from the case, the day shift and the night shift, the families at the ball, and put a small prize on the first correct answer. Teams cannot share evidence, but they can bluff. The birthday person floats between tables as a consultant, which keeps them at the center of the party.
Paper makes better photographs. If the birthday person likes a table covered in evidence, print the PDF of a case, use clips and folders, and put a red string board on the wall for the timeline. If you are hosting in a bar or a busy apartment, phones are easier. A mix works too: play online, but print the autopsy report and the map so there is something to hold.
In an online room, everyone votes in secret and the host, the birthday person, reveals all the answers at once. Then open the solution file and read it aloud like the last chapter of a novel. If the group got it wrong, it does not matter much: the case closes either way, and hearing the solution file explain what everyone missed is half the fun. If they got it right, that is your toast.
If the evening goes well, a yearly pass to every case is a good present for the person who loved it most. It costs $69.99 and gives them a reason to host the next one.
At some point in the evening the room will go quiet and someone will suggest looking for hints. There are none, by design. Instead, try the host's reset: everyone reads out the single document they trust most and the single one they trust least. Disagreement about which records are reliable usually breaks the deadlock within ten minutes, and it gives the birthday person a natural moment to take charge again.