To be clear, there is no longer a ship-wide lobster night. The Captain's Gala menu is served on three nights as the alternate menu for the Royal Court restaurant. So, when you visit that restaurant for the second time (or third, if you have Pirate Night there), you'll have the lobster menu.
If you already know your dining rotation, then you can figure out which night will be lobster night. Otherwise, you'll have to wait until you get your rotation.
The ship-wide thing stopped many years ago.