If POTUS wants to use 10 USC 284 to build the Wall - minimum 15 days notice needs to be given to Congress.
Possibility that DJT reopened government for that waiting period
Letter from Dem senators to Mattis (Dec 2018) re: Wall funding
They are particularly concerned with the use of 10 USC 284 & 2808
https://www.reed.senate.gov/news/releases/democratic-senators-urge-pentagon-not-to-use-defense-funds-for-border-wall-along-arizona-bombing-range