Arsal385 wrote:17th March is the earliest date for both applicants. Since your son got his ILR on 17th March 2015 so 17th March 2016 is the first anniversary of his ILR and at the same time you can avoid new naturalisation fee which is valid from 18th March 2016 onwards. Remember date of NCS appointment is the date of HO receive your application. Also make sure you and your son were physically present 5 years prior to the date of application submission i.e. on 17th March 2011.
Your comment above is confusing. Rephrased it should state that 'the date of your NCS appointment is the date of application'.
To the OP, you can apply already as you got ILR on 4th March. Your dependents on 17th as stated. Unlikely you will get an earlier NCS appointment as a few members are posting that they can't get one before 18th in many areas.
Of course you all need to make sure that you were physically present in the UK on the exact same date 5 years ago.