it does not matter you buy bitcoin at this time even though after you buy the bitcoin price continues to decline, you just have to hold and wait until the end of the year.
I think that this can be consider the best time to invest your money in bitcoin, because bitcoin price is trading too much stable at the current price for along time and i am sure that after that it will start increasing its price therefore i think that we should try our best to invest money in bitcoin in current time and make our position more stronger.