Red Cabbage is a vegetablecrop that grows from Red Cabbage Seeds after 9 days. The seeds are available starting in Year 2. There is a small chance of buying Red Cabbage or Red Cabbage Seeds from the Traveling Cart. If choosing "Guarantee Year 1 Completable" from Advanced Options when starting a new game, the Traveling Cart will sell a Red Cabbage Seed sometime during year 1.
Because the only way to easily get Red Cabbage or Red Cabbage Seeds in year 1 is the “Guarantee Year 1 Completable” option on the starting screen, people have devised the best way to get the vegetable or the seeds in year 1 without the option, primarily for mobile (1.4.5) users. Options available are:
Drop from Purple Slimes (0.5% chance - works pre-1.5)
Drop from Mummies (0.2% chance - 1.5 only)
Drop from Serpents (0.2% chance - 1.5 only)
Treasure Rooms in Skull Cavern (≈0.0015% chance - works pre-1.5 but is extremely rare [1 in 675])
Rarely at the Traveling Cart (not usually found in Year 1 - works pre-1.5, seeds or vegetable)