Clarification on when to apply :0

I know this question is probably asked a bunch on here, but can someone just clarify this for me pls? I feel like I'm seeing mumbo jumbo and not a straight answer and it's confusing to me 😭.

I know vet school applications open in January, and close in September. But when would you start your application if you're in your undergrad and don't want to take a gap year? Would you start your application in January of your 3rd year of undergrad or would you start it in January of your senior year? Is it like high school where you apply in the fall of your senior year to go straight to college without a gap year? Thank youu