What are Zero Cross Detection Advantages and algorithm to implement it??

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Hello!! Everyone

In a system, i have to switch some relays, and i had read somewhere that it is advised to switch the relays ar Zero-Cross Point of AC Signal, may i know the reason behind this..
Apart from that, how to detect this using a micro-controller (I am using an Opto-Coupler circuit with an interrupt pin to detect ac signal, circuit is like that at zero cross point it will output 5V and at rest 0V).

What is the best algorithm to implement this (switching relays on zero cross point).

There are two Zero Cross points one is High to Low and another one is Low to High Points, so which one to use.

I am using ATmega2560 micro-controller and have to switch relays, please help me in writing a program with which i can detect zero cross point and then properly switch my relays.
 

Presumed we are talking about mechanical relays or "contactors" (larger power relays). Theoretically you would want to switch a resistive AC load on or off during voltage zero crossing to reduce arcing. Practically, relays have considerable delay, often larger than 10 ms, so if you power the relay coil at zero crossing, the contact may close just at voltage maximum. Target missed.

For inductive loads, there's no purpose in switching at zero crossing.

Without exact knowledge about relays and load, it's simply useless.
 
I am using this relay.

https://www.mouser.com/ds/2/89/9000_Spartan-6107.pdf

I don't know what exactly is the use of this, but as per my knowledge, it will used to turn on heater element.

So my load is an resistive load and relay type is already mentioned.

So how will i switch relay to reduce arcing, my frequency and Voltage is 50Hz, 230 V.
At Zero Cross point i am getting 5V and at other points i am getting 0V at the output of the circuit, which i am using to sense the Zero Cross Point.
This output is connected to Port Change Interrupt pin of the micro-controller.
 

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