Historically, the market is not kind to a president in the first 30 days after the inauguration. In the table below, which covers the last 11 presidents, the S&P-500 declined an average of -2.59% in the first 30 days of a presidency.
This does not take into account any pre inauguration market gains and losses. Given our recent post election gains, it would suggest any decline could be intensified if it appears Trump's promises/policies are not going to be immediately implemented. There is a lot of risk that many of them will be blunted severely and/or face a tough uphill battle.