How to Make Players Deal More Damage at Night?

Setup a scoreboard with the current time: scoreboard objectives add currentTime dummy

In a tick function or repeating command block put: execute store result score @a currentTime run time query daytime

Under the above line in your tick function or in a different repeating command block: execute as @a if score @s currentTime matches 13183..22800 run effect give @s minecraft:strength 1 0

Basically, when the time is night it will give every player strength 1, which will make them hit harder. You can change the level of strength from 0 to whatever you want depending on what damage boost you want.

You could always have an inverted daylight sensor that powers a repeating command block that requires redstone. I’m it have the command /effect @a strength 9999 1 (for strength 1). Then have a normal daylight sensor that has a repeating command block that requires redstone. Then the command would be /effect @a clear. If you get too far it will stop working, but you could always build more. (The commands I wrote are for pocket edition, I don’t know them for java edition).