Since I’m pressed for time, let’s jump right into this, shall we? The biggest change in place (besides the ongoing 7 line shutdown) is the closure of the Lenox Avenue line, which will be in effect for the next three weekends. The 3 is completely suspended for the entire weekend (4 trains are running local in Brooklyn to/from New Lots Av as a replacement). 2 train service is split, running from 241 St to 3 Av-149 St and from 96 St to Flatbush Av. Shuttle buses will serve the intermediate stations. There’s also no 5 train service between East 180 St and 149 St-Grand Concourse.
Also in effect this weekend are express runs of the 6, N and R trains in Manhattan and local runs on the A and E & F in Brooklyn and Queens respectively. See the map for details. Enjoy the weekend.
Saturday EDIT: As Amtrak7 pointed out in the comments, there are four shuttle buses running for the Lenox Avenue shutdown. The local buses from 96 St/Broadway run to both 148 St-Harlem and 3 Av-149 St. Local buses from 3 Av-149 St run to 149 St-Grand Concourse as a one-stop shuttle as well as 96 St/Broadway. Watch the signs on the buses to ensure the bus you’re taking is heading to your destination.
Also, uptown 5 trains heading for 149 St-Grand Concourse skip 138 St. The map has been updated to show this.