Who despises the day of small things? Men will rejoice when they see the plumb line in the hand of Zerubbabel.
The "day of small things" refers to times when little progress is apparent for God’s people. In context, the author was speaking about the rebuilding of the Temple following the Babylonian captivity. But most of life is a matter of small things, of incremental steps. The writing of a novel does not happen in a day. Instead, it is like building a wall, brick by brick, or like eating a cow, something all of us have done, but its been one hamburger at a time. If you write one page a day, in a year you have a book.
Whatever it is that God has you doing, just keep at it. Perseverance over time is the key. Don't despise the day of small things. When Jesus was born in Bethlehem, he was just one more tiny infant born to unwed parents, a speck in the vast teaming mass that made up the Roman Empire. But in the fullness of time, he was the Savior of the world.