I am not an specialist in History of Economic Thought, but as someone that teach Macroeconomics I believe that there is a lot of confusion about the real meaning of the Say´s Law. For other side, its look pretentious thinking that Keynes and a lot of your direct folower do not have understood correcltly the classical framework. Nevertheless, that is no a question for Macroeconomists thats doing theory from the neo-consensus macroeconomics (New-Keynesian DSGE models).

According to my opinion, neoclassical synthesis can be examined for this subject.

Quite the opposite. In order to understand Keynes you should have previously studied the old classical approach and understood it. Of course, this doesn´t mean you are obliged to share the classical´s points of view. However, one thing is what Keynesians say, another one what Keynes himself said. I guess the difficulty is with many so called Keynesians who do not understand neither classical economics nor Keynes´s original theory.

The classical economists could not explain either persistent high unemployment in the UK in the twenties and neither the increase after 1929. Nominal wages were lowered throughout this period but, contrary to classical convinctions, this neither led to lower labour supply or higher labour demand. The anomaly increased when unemployment did decline fast after the UK went of gold in october 1931. Keynes tried to explain this and needed a general theory of the economy which encompassed the classical case but which encompassed other states of the economy. The crucial variable (chapter 2 of the General Theory) was the (for classical economists) new concept of involuntary unemployment or unemployment which persisted when the outputprice/labour ratio increased (compare someobody sitting at home with a zero hours contract who gets called to work for two days but not for the entire week as his boss was able to get a customer by cutting corners with regard to the amount of hours this project wold really take). Keynes his idea about this was that an economy knows different kinds of spending (investments, consumption, the government, exports and imports) and that for instance a sudden decline in investment, which (everybody agrees) will lead to a decline in employment, an increase in unemployment and downward pressure on wages won't be compensated by rising consumption as lower wages will depress consumption, surely when unemployment is high and expectations are dismal. As consumption does not recover, investments will stay in the doldrums too, even when interest rates are low (hey, are you really going to invest, even when interest are low, when you have slack capacity...). As a result, unemployment will stay high. Seeking harder for a job will only lead to crowding out of order workers and lower wages will only lead to less consumption (Keynes advocated a policy aimed at keeping the purchasing power of the money wage, instead of money, intact). The whole 'aggregate' thinking behind this in combination with the interplay between the sectors was alien to classical economists and was of course inspired by the new National Accounts, which mapped investments and consumption. Lucas c.s. tried to negate these ideas by introducing the idea of 'complete markets' into macro economics, i.e. the idea that prices are not just caused by present day transactions but also by transactions next year and in fact forever after. This explains unemployment by stating that it is the consequence of the idea that you will unexpectedly be more productive tomorrow, so you have to work less today (really). For a Keynesian economist it is indeed hard to understand 25% unemployment in Greece (where wages decreased with 40%) as a consequence of such future oriented complete markets.