This is what I did:
1) Downloaded the clam client. It was taking forever so I downloaded the block chain snapshots. This took a few days.
2) Emptied out my Bitcoin wallet (transferred all the coins to my Trezor hardware wallet).
3) Imported my Bitcoin wallet into my Clam wallet.
4) Got a few clams because some of the addresses in my Bitcoin wallet had value on the distribution date.
Pretty easy.
There is a lot of information about this if you just search for it. Try Google. That is the search engine I use.
why only on that particular day there is a reason behind this?
Because it is by far the fairest and most efficient way to create an initial distribution of coins. If you have an idea for a better way you can, of course, create your own coin using your method.