28 days before qualifying date

[Et1234]

Hi
My full 5 years to apply ILR falls on 26/4/2018 inclusive.
To avoid fees increases that usually take place in early April, I'd like to make use of the '28days before' period to apply early. I worked out it falls on 30/3/18 and I checked it will be Good Friday and Easter/ Bank holidays that few days along that week.
So, my question is what date is the HO looking at to determine the 28days?
Is it the date we post the application (I am using postal method) or the date HO receive the application at their end?

Pls advise and thanks !!

[Casa]

The date of posting.