In Zechariah 7 we read that the people of Bethel sent messengers to inquire about mourning and fasting in the fifth month. There were three main holy days commanded by God.There are no commanded days in the fifth month.  The people had been mourning the destruction of the temple by Nebuchadnezzar on the anniversary of that national catastrophe.  

God answers with a few questions.  They were fasting, but God questions if they were fasting for Him or for themselves.  Then God addresses some of the feast days and questions again if it was for Him or for themselves.  

Zech 7:5-6
"Ask all the people of the land and the priests, 'When you fasted and mourned in the fifth and seventh months for the past seventy years, was it really for me that you fasted? And when you were eating and drinking, were you not just feasting for yourselves?Ouch! Sounds a bit like some of our “Christian” holidays today.

Later on in Zechariah 8 God announces that the fasts will no longer be sad occasions of mourning some national catastrophe but will be occasions of joyful happy festivals. 

When the temple was completed, the people of Israel celebrated with great joy. Shortly afterward, they celebrated the Passover. They did everything according to the Book of Moses.

Ezra 6:21
So the Israelites who had returned from the exile ate it, together with all who had separated themselves from the unclean practices of their Gentile neighbors in order to seek the LORD, the God of Israel.

At this time the Israelites did not conform to the practices of the nations around them, they followed God’s commands in honoring and celebrating His holy days, in the way He commanded. They weren’t eating and drinking and feasting for themselves as pointed out in Zechariah 7:5-6. What lessons can we learn from this?