8 Then the word of the Lord came to Zechariah, saying, 9 “Thus says the Lord of hosts:
‘Execute true justice, Show mercy and compassion Everyone to his brother. 10 Do not oppress the widow or the fatherless, The alien or the poor. Let none of you plan evil in his heart Against his brother.’
Zechariah 7:8-10
Here we see what God sees as important - the true justice is showing mercy and kindness to your brother. Not oppressing the widow, the fatherless, the alien or the poor and to not plan evil in your heart.
This is all about kindness to your fellowman. Especially this kindness should be shown to your family and to those who are down and out, suffering.
This was spoken to the Jewish people who had been in bondage in Babylon for 70 years. They should have been able to understand what showing kindness to those suffering would mean because they themselves had suffered.
The basis of what Jesus said in the New Testament, do unto others you would have them do unto you. (Matthew 7:12/ Luke 6:31).
If you look at your fellowman and see them suffering and feel no compassion, then sacrifices such as fasting is meaningless.