It appears the decline on 12/19/17 was probably wave "IV" of a post Triangle thrust up.
There is usually a Fib relationship between waves "I" and "V".
In this case wave "I" which is from 2632.71 to 2671.88 = 39.17 x .618 = 24.20 + 2680.74 wave "IV" targets SPX 2704.94.
Allow for leeway plus or minus two points.
If US Senate vote on the tax bill happens during market hours, an SPX top could occur within minutes of completion of the vote.