I just coined the phrase recently. It means having a day where you get half of your stuff done- but its totally useless and frustrated because you can't find or do the other half.
Hence the term, "one sock day".
Don't you just hate it when you can't find the other half of your socks? I've wasted so much time trying to find them. God knows where they go.
The funny thing is that when I came back to Singapore, I found one of my missing sock.