In Ezra 3, we see that the work began. They began to rebuild and to sacrifice and keep the feasts and give. Then, in the second year and the second month, they began to rebuild the temple, and when the foundation was done, there was great singing and rejoicing and praising God and giving thanks!
There were some that were praising God because of what they could do now - they could go on to build the rest of the temple and continue on with the great future that lay before them. Praise God for what is ahead of you. God already has the future!
There were some that were rejoicing for the present. The foundation was laid before their eyes. They were making progress and working together right then and there, and they were unified as one man (verse 1) to accomplish this. It was a good time to be alive! Rejoice in what God is accomplishing right now. Appreciate those around you. Relax and serve God, thanking Him for what He has for you in "the now."
And then there were the "ancients" - those who had seen the first temple, and now were here to rebuild it again. They wept and thanked God for His mercy; for bringing them back out of judgment; for bringing them through to where they could serve Him again. They remembered the "glory days" and could see that the God of the past is the God of the present and the God of the future.
Verse 13 tells us that if you were there, you would not be able to discern the weeping from the joy and the shouting. God is good! Give Him glory for the past, serve Him in the present, and seek Him for the future. His mercy endureth for ever!
(Ezra 3:11-13) "And they sang together by course in praising and giving thanks unto the LORD; because he is good, for his mercy endureth for ever toward Israel. And all the people shouted with a great shout, when they praised the LORD, because the foundation of the house of the LORD was laid. (12) But many of the priests and Levites and chief of the fathers, who were ancient men, that had seen the first house, when the foundation of this house was laid before their eyes, wept with a loud voice; and many shouted aloud for joy: (13) So that the people could not discern the noise of the shout of joy from the noise of the weeping of the people: for the people shouted with a loud shout, and the noise was heard afar off."