８日(ようか)は猫を食べました
You could even omit the は.
#日 means Xth day of the month (as long as you use the proper pronunciation), so you don’t need to add anything else.
#日目 would be like the Xth day, like “the second day of school”.
第#日 would be like Day X, “day 1; week 2; lesson 5; etc.”