Bayern Lander Train Tickets

Yes, just buy it from a machine in Munich. There are no constraints on the sale of these tickets, no price advantage in purchasing now. The DB ticket machine allows you to specify the date of use. Your signature is required on the ticket - the conductor will handle any stamping.

"must I specify which day we are planning to use it" It depends on the automat. I have purchased Bayern-Tickets where the automat asked me for the date, and it was printed on the ticket. No further date stamping was necessary. However, I have seen Bayern-Tickets from other automats, presumably without date stamping ability, that had to be date stamped. You can identify these tickets by the words, "Hier entwerten" printed on them with arrows to show where you put them into the stamping machine. And the signature has to be in "Block Schrift" (printed). It will probably ask you for 1) Bayern-Ticket, 2) Bayern-Ticket Single, or 3) Bayern-Ticket Nacht. You want #1. Then it might ask you for 1) Heute (today), 2) Morgan (tomorrow), or 3) Andere (other). If you say Andere, it will show you a calendar. Or it might just start with the calendar (no Heute, Morgan, etc) Bayern-Tickets are valid not only for regional trains in Bavaria, but also for all modes of transport in the Metro district, MVV. So yes, you can use it on the S8 from the airport to Ostbahnhof, or for the S1 through the Hbf to the Ost Bahnhof. The LH track in the station is for S8; the RH track for S1. You will undoubtedly arrive in Munich at T2. Walk out the front door and across the tent covered plaza to the S-Bahn station, which you will recognize by the big green circle S. Go down the stairs to the platform and there will be several ticket automats. Find the button for Bayern-Ticket. There might also be one for "Regio-Ticket Bayern". That's NOT it.