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 Berkely

Statistical and Applied Mathematical Sciences Institute (SAMSI), North Carolina