It means the first day of TIME. Before the beginning, there was no time, no space, no matter.
In the beginning — of time
God created the heavens — space
And the earth — matter
None of the three can exist without the other two. There had to be space for the matter to occupy. There had to be matter for time to exist, as time is the measurement of change, and if there is no matter to change, then there can be no measurement of change. And there cannot be matter if there is no space to put it in.
This is why one sentence explains it all… time, space, matter.
All created at the same… moment, the FIRST moment.