How do you make Zombies spawn at night and die in the day?

Assuming that you have a functioning day/night script already made, you can use the GetPropertyChangedSignal event with the ClockTime property of Lighting to achieve what you're looking for.

Once you've connected this event, you can use a debounce to keep the function from firing in later portions of the day.

An application of this can be found below:

local LightingService = game:GetService("Lighting")
local TIME_DAY = 6 -- Set this to the hour you want the zombies to die
local TIME_NIGHT = 18 -- Set this to the hour you want zombies to spawn
local zombiesActive = false -- This is our debounce

local function ClockTimeChanged()
    local newClockTime = LightingService.ClockTime

    if zombiesActive then
        -- If zombies are out, lets check if the ClockTime is day.

        if (newClockTime >= TIME_DAY and newClockTime < TIME_NIGHT) then
            zombiesActive = false

            -- PUT YOUR CODE TO DESPAWN THE ZOMBIES HERE
        end     

    elseif (newClockTime  >= TIME_NIGHT or (newClockTime > 0 and newClockTime < TIME_DAY)) then -- If zombies aren't spawned, we check if the ClockTime is night to spawn them.
        zombiesActive = true

        -- PUT YOUR CODE TO SPAWN THE ZOMBIES HERE
    end
end

LightingService:GetPropertyChangedSignal("ClockTime"):Connect(ClockTimeChanged) -- This is where we are making the connection to listen to when the time is changed. Once the time changes, the ClockTimeChanged function will fire.

The rest here is up to you. I've left comments in caps as indication of where to put your code.

If you have any questions, be sure to ask. Best of luck!