Besides prices that "gapped" down by at least 55 pips during the opening on Monday, you also have to consider the swap/roll-over costs: http://alpari.com/en/faq/trading_terms/overnight_swaps/
Alright thank you FMIC, I was not aware of that type of commissions, I have only seen that happen on weekends maybe it's different on real money accounts.
Orders held over 5 PM NY time have swap. Some brokers don't use the OrderCommission and OrderSwap but instead have entries in history (e.g. FXCM.)

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Maybe this is a basic question but I can't figure this out
Why did my equity decreased more than usual from Friday 23:55 to Monday 00:00
Is there a commission I am being charged with? I am on Alpari demo server
According to my C++ simulation the equity at 00:00 should have been 9882.11 instead of 9880.83
strategy tester is configured with a fixed spread of 12 pips on both
currently my formula for equity is balance + each order Bid-open price
...before (test.cpp:65) equity+= -5.4 ( 1.20003 - 1.20543 )* 0.01 * 100000 (test.cpp:65) equity+= -4.26 ( 1.20003 - 1.20429 )* 0.01 * 100000 (test.cpp:65) equity+= -6.36 ( 1.20003 - 1.20321 )* 0.02 * 100000 (test.cpp:65) equity+= -8.43 ( 1.20003 - 1.20284 )* 0.03 * 100000 (test.cpp:65) equity+= -4.08 ( 1.20003 - 1.20105 )* 0.04 * 100000 (test.cpp:65) equity+= -2.15 ( 1.20003 - 1.20046 )* 0.05 * 100000 (test.cpp:69) total equity 9969.95 (strategy.h:68) 2015.01.02 23:50 Bid 1.20003 Ask 1.20015 balance 10000.63 equity 9969.95 margin 38.4604 freemargin 9931.4896 (test.cpp:65) equity+= -5.32 ( 1.20011 - 1.20543 )* 0.01 * 100000 (test.cpp:65) equity+= -4.18 ( 1.20011 - 1.20429 )* 0.01 * 100000 (test.cpp:65) equity+= -6.2 ( 1.20011 - 1.20321 )* 0.02 * 100000 (test.cpp:65) equity+= -8.19 ( 1.20011 - 1.20284 )* 0.03 * 100000 (test.cpp:65) equity+= -3.76 ( 1.20011 - 1.20105 )* 0.04 * 100000 (test.cpp:65) equity+= -1.75 ( 1.20011 - 1.20046 )* 0.05 * 100000 (test.cpp:69) total equity 9971.23 (strategy.h:68) 2015.01.02 23:55 Bid 1.20011 Ask 1.20023 balance 10000.63 equity 9971.23 margin 38.4604 freemargin 9932.7696 (test.cpp:65) equity+= -10.89 ( 1.19454 - 1.20543 )* 0.01 * 100000 (test.cpp:65) equity+= -9.75 ( 1.19454 - 1.20429 )* 0.01 * 100000 (test.cpp:65) equity+= -17.34 ( 1.19454 - 1.20321 )* 0.02 * 100000 (test.cpp:65) equity+= -24.9 ( 1.19454 - 1.20284 )* 0.03 * 100000 (test.cpp:65) equity+= -26.04 ( 1.19454 - 1.20105 )* 0.04 * 100000 (test.cpp:65) equity+= -29.6 ( 1.19454 - 1.20046 )* 0.05 * 100000 (test.cpp:69) total equity 9882.11 (strategy.h:68) 2015.01.05 00:00 Bid 1.19454 Ask 1.19466 balance 10000.63 equity 9882.11 margin 38.4604 freemargin 9843.6496 ...equity is wrong from here (test.cpp:65) equity+= -12.94 ( 1.19249 - 1.20543 )* 0.01 * 100000 (test.cpp:65) equity+= -11.8 ( 1.19249 - 1.20429 )* 0.01 * 100000 (test.cpp:65) equity+= -21.44 ( 1.19249 - 1.20321 )* 0.02 * 100000 (test.cpp:65) equity+= -31.05 ( 1.19249 - 1.20284 )* 0.03 * 100000 (test.cpp:65) equity+= -34.24 ( 1.19249 - 1.20105 )* 0.04 * 100000 (test.cpp:65) equity+= -39.85 ( 1.19249 - 1.20046 )* 0.05 * 100000 (test.cpp:69) total equity 9849.31 (strategy.h:68) 2015.01.05 00:05 Bid 1.19249 Ask 1.19261 balance 10000.63 equity 9849.31 margin 38.4604 freemargin 9810.8496