Sunday: During the day you can't beat Rio in Los Yoses (San Pedro). 2 for 1 pitchers of Imperial all day long. At night, hands down the best bar is Bulldogs in San Francisco de Dos Rios. The bar get's absolutely packed so be sure to get their early if you want a seat. Head down the street a little ways to Fiesta Mexicana if you want a seat with good food and booze for cheap.
Monday: Every young foreigner in San Jose will be at El Cuartel de la Boca del Monte, guaranteed. El Cuartel is actually two separate bars that are combined on Mondays to accomodate the crowd. Bar service is good and the live music is impressive.
Tuesday: Tuesday is quite possibly the slowest night in the metropolitan area for going out, but there seems to always be a decent crowd at Rhapsodia on Paseo Colon (San Jose), or La Calle de la Amargura in San Pedro were there is a bar every 10 feet.
Wednesday: head to Lubnan in Paseo Colon for a people packed night of house and electronic music from local DJs. This tiny little Lebanese restaurant not only has awesome food, but the bar in the back has a speakeasy feel to it, and one wonders when entering through the unmarked back alley entrance if they even have a permit for the bar. Late night head to El Pueblo until the sun comes up.
Thursday: Lots of options on Thursday nights. Qbico in Los Yoses, Retrovision in San Pedro, La Calle de la Amargura, Jazz Cafe (San Pedro) and just about any bar in Santa Ana, Tibas or Heredia. Van Goh in Sabanilla or in Tibas are good as well.
Friday: Everywhere, period.
Saturday: On Saturday nights, Esczu is packed with young faces. All the way from the highway to Santa Ana, you'll have your choice of bars. The most packed bar is probably 5 on Saturday nights. You can try Mas Tequilla or Henry's as well.
Other Awesome Bars:
Olio - Barrio Escalante - Great wine bar and awesome mediteranian food
Los Parales - Curridabat - Cheap cheap cheap bocas that are worth the 40 minute wait some nights
Vyrus - Los Yoses - Awesome rock bar
Nova - Los Yoses - Clubbish bar, heavy handed drinks
Sand - San Pedro - Hardcore rock bar, wear your black
Rolling Stones - Tibas - Awesome dive bar