There are several S-Bahn lines that can get you straight from Berlin to Potsdam. It is very easy if you just look up transit lines out of Berlin.

The Guide Books we have been using mention "OPEN Apr.-Oct. 10am-6pm Tue-Sun", but the German language german website spsg listing castles and gardens mentions "Opening times Year round daily from 8 o clock till dusk, Valid 01.01.2016 - 31.12.2016 Monday-Sunday: open all day". To avoid disappointment it may be better to avoid the Monday or to have someone check by telephone about the Monday opening if that is the only day you could go. If you want to visit the palaces (especially Sans Souci and the New Palace) you will need a ticket (you can buy a ticket valid for all castles) but in addition you will need a time slot for each castle separately. We were unable to find an online way to purchase timeslot reservations. Good luck and enjoy your stay!

Nosotros la realizamos en una excursión guiada en castellano que sale desde la torre de televisión los martes, jueves, sábado y domingos la empresa se llama Cultourberlin y realiza otras excursiones, los podes identificar xq están vestidos siempre con algo de color verde

I believe Sanssouci Palace will be close but Sanssouci Park will be open. The Park is amazing so you will enjoy it very much. For more information please refer to their website (can't post a link in here sorry)

Yo lo hice con mi hija simplemente tomando un tren desde Berlín en la estación del metro y en la estación de Postdam nos ofrecieorn el city tour estilo hop on hop off en donde uno mismo decide a qué entrar y a qué no. Toma el día entero. Entiendo que por esta época que está haciendo mucho frío no ofrecen los tours en bicicleta. Es muy manejable.