Taharos, Chapter 6, Mishnah 7. This chapter is occupied with defining which places count as a reshus harabbim and which count as a reshus hayachid, since a sfeik tumah is ruled tahor in the public domain and tamei in the private domain. A theme that has already surfaced is that one and the same place can hold two classifications at once, private with respect to Shabbos and public with respect to tumah. The reason is that the two systems are built on different foundations: the domains of tumah are derived from the parashah of sotah, so what counts is seclusion rather than ownership and walls. A secluded place is a reshus hayachid; a place where people are constantly coming and going is a reshus harabbim, because that is not a setting in which a woman could be suspected of adultery. Our mishnah now shows that a single location can even switch classification with the seasons.
The Bik'ah
"Habik'ah": the valley, that familiar expanse of open farmland that appears and reappears throughout Shas. The mishnah rules that its status depends on the time of year.
"Bimos hachamah, reshus hayachid l'Shabbos u'reshus harabbim l'tumah": during the sunny season, when everyone is out in the fields planting and working, it is a private domain regarding Shabbos and a public domain regarding tumah. Note that the term reshus hayachid here is being used loosely; strictly speaking such an area is a karmelis, treated by the Chachamim as a public domain for Shabbos, though by Torah law it is not a reshus harabbim. Regarding tumah, however, it is plainly a public domain, since a field crowded with workers walking about is anything but a hidden, secluded place.
"Uvimos hageshamim, reshus hayachid l'kach ul'kach": but during the rainy season it is a private domain for both purposes. For Shabbos that is clear enough, and now it holds for tumah and taharah as well, because in that season there are not many people out farming, and the valley becomes the sort of empty, secluded expanse that qualifies as a private domain.
So the very same valley yields opposite rulings in a case of sfeik tumah depending on the calendar: in the sunny months the doubt is resolved leniently, as in a reshus harabbim, and in the rainy months it is resolved stringently, as in a reshus hayachid.