Here is a list of institutes in math that host many interesting workshops and/or short courses.
Fields Institute for Research in Mathematical Sciences, University of Toronto
Institute for Computational and Experimental Research in Mathematics (ICERM), Brown University
Institute for pure and applied mathematics (IPAM), UCLA
Institute of Advanced Study (IAS), Princeton University
Institute of Mathematics and its Applications (IMA), University Of Minnesota
Simons Institute for the theory of computing, UC Berkeley
Simons Laufer Mathematical Sciences Institute, UC Berkely
Institute for Mathematical and Statistical Innovation (iMSi), University of Chicago
Center of Mathematical Sciences and Application, Harvard University