There is a lot going on this Summer (or Winter, if you head to Brazil). I have compiled a list of schools that seem very interesting for those who like algorithms, optimization, and related topics. Some have no registration available yet. The deadline for two of them are over and for a few others it is approaching. However, because some of those are (or could be) recurrent, I felt that listing them would be useful if someone reads this in the future.
International Summer School on Autonomous Agents and Multiagent Systems
May 6-8 (deadline: April 15)
Singapore
4th International Symposium on Combinatorial Optimization (ISCO) Spring School
May 19-20 (deadline: April 12)
Salerno, Italy
* Included on March 10 by suggestion of Martim Joyce-Moniz and Daniel Oliveira
117th European Study Group with Industry
May 23-27 (deadline: May 27)
Avignon, France
* Included on March 10 by suggestion of Martim Joyce-Moniz
Integer Programming and Combinatorial Optimization (IPCO) 2016 Summer School
May 30-31 (deadline not posted yet)
Liège, Belgium
Eötvös Loránd University Summer School in Mathematics: Discrete algorithms and applications
June 6-10 (deadline: May 31)
Budapest, Hungary
COST/MINO PhD School on Advanced Optimization Methods 2016
June 6-10 (deadline: May 13)
Rome, Italy
* Included on April 7
International Conference on Automated Planning and Scheduling (ICAPS) Summer School
June 8-11 (deadline: March 18)
London, UK
Numerical Computations: Theory and Algorithms (NUMTA) International Conference and Summer School
June 19-25 (deadline: March 10 for regular fee)
Calabria, Italy
Association for Constraint Programming (ACP) Summer School 2016
June 20-24 (deadline: May 20)
Cork, Ireland
Mixed Integer Non-Linear Programming (MINLP) School: Theory, algorithms and applications
June 20 – July 1 (deadline was February 20)
Sevilla, Spain
Satisfiability (SAT), Satisfiability Modulo Theories (SMT), and Automated Reasoning (AR) Summer School
June 22-25 (deadline not posted yet)
Lisbon, Portugal
2016 Summer School on Real Algebraic Geometry and Optimization
July 11-15 (deadline was April 1st)
Atlanta, GA, United States
* Included on June 9 by suggestion of Akshay Gupte
2016 Industrial Math/Stat Modeling Workshop for Graduate Students
July 17-27 (deadline: April 15)
Raleigh, NC, United States
* Included on March 20
São Paulo School on Algorithms, Combinatorics, and Optimization
July 18-29 (deadline: March 28)
São Paulo, Brazil
12th Multipe Criteria Decision Aid and Multiple Criteria Decision Making (MCDA/M) Summer School
July 18-29 (deadline was December 30)
Recife, Brazil
Argonne Training Program on Extreme-Scale Computing
July 31 – August 12 (deadline: March 25)
St. Charles, IL, United States
Prague Summer School on Discrete Mathematics 2016
August 1-5 (deadline: March 31)
Prague, Czech Republic
3rd Algorithmic and Enumerative Combinatorics (AEC) Summer School 2016
August 1-5 (deadline: June 15)
Hagenberg, Austria
Institute for Mathematics and its Applications (IMA) Program on Mathematical Optimization
August 1-12 (deadline not posted)
Minneapolis, MN, United States
* Included on March 10 by suggestion of Jeff Linderoth
* Website link added on March 14
Statistical and Applied Mathematical Sciences Institute (SAMSI) Optimization Program Summer School
August 8-12 (deadline: June 8)
Raleigh, NC, United States
* Included on March 24 by suggestion of Sercan Yildiz
Algorithmic Optimization Autumn School
September 19-22 (deadline: August 21)
Trier, Germany
* Included on May 10
Thiago Serra 
Great list! I did not know about the São Paulo School. Thank you.
Does a spring school qualify for the list?
http://www.isco2016.it/spring-school (May 19-20, 2016)