Post-Sandy repair work on the 4/5 tunnel connecting Brooklyn and Manhattan resumes this weekend, so there will be no service in either direction on the 4/5 platforms at Borough Hall from 11:45 p.m. tonight (Friday, January 27) and 5:00 a.m. Monday, January 30. 2, 3, and R trains will, barring unforeseen problems, provide alternative service. Also over the weekend, once again Manhattan bound F trains will be diverted to the A/C line between Jay Street and West 4th Street, which means no Manhattan bound service at York Street in DUMBO. To get to Manhattan from there, walk uphill to the High Street A/C stop. To get to the LES stops on the F, take an A, C, or F to West 4th and catch a Brooklyn bound F there. Finally, on Saturday and Sunday, January 28 and 29, there will be no C train service in either direction. A trains will, however, provide normal service, barring anything unforeseen.
The 4 train tunnel work will continue the following work week, but only late at night 10:00 p.m. to 5:00 a.m. Monday, January 30 to Friday, February 3. 2 and 3 trains will provide alternative service, and A and N (replaces the R late at night) trains should be running normally.
For service changes that may affect travel on other parts of the system, either during the weekend or the following week, see MTA Info or The Weekender. Especially for those traveling into or from Brooklyn, I encourage you to check the MTA site; there are many changes that could affect your trip.