Capital gains tax on development agreement

Hi,

I entered into a development agreement in March 2018 for a builder to construct apartments on my site and the % split will be 50:50. The builder expects to complete the construction in August 2019. The land was in my name since 2013. When is the right time for me to sell the apartments to attract long term capital gains tax only? Is it before getting the completion certificate or after getting the completion certificate from competent authority? Please note that the actual sale value will be much more than the Govt. registration value. Thanks,