Taken from Ryanairs website......
Flights TO and FROM Riga (RIX/Latvia) and Kaunas (KUN/Lithuania) for travel within 7 days of departure & flights TO and FROM Berlin Schonefeld (SXF/Germany) for travel within 3 days of departure can only be booked by calling our reservation centre.
.... Thats why it showed the flight on Monday as being fully booked.
However in my experience they will take as much return trip fuel as possible with them to certain destinations. Sometime this is for cost and other times its because of possible problems with fuel supplies down-route.