If I buy ETF twice a year fixed date for rebalancing, but if market free-fall but my next buy is still 6 month later, what would be your advice?

Ah tough question. Usually i would advocate you see the magnitude of the fall. if the fall is 10% or 20% its not too bad, but if you think this has reached 50%, then may be you want to put both your contribution for the year in at once.

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if you do not have the money, then the decision is obvious. there is nothing much you could do.

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if its a 10-20% draw down and prior to this there were some gain, your allocation should be still pretty intact. however if the draw down is 50%, your equity portion could be smaller, the rebalancing could be to sell some of your bonds and move them to equity.