When bars start switching out their taps from Sam Adams Summer Ale to Octoberfest, you know it’s fall. Learn more about how such beer is brewed at the Samuel Adams Brewery in Jamaica Plain, which holds free tours ($2 suggested donation, proceeds benefit local charities) Monday through Saturday. Not a Sam fan? Harpoon Brewery on Northern Avenue offers weekend tours and weekday beer tastings. Tours are $5 and tastings are free. And for brew lovers who can’t make it to Germany this fall, check out Harpoon’s 23d annual Octoberfest celebration Sept. 28 and 29, with beer, food, and music. Admission is $20.