Snapshots for WCT are taken throughout the entire month, and your distribution is based on the average amount of coins held in your wallet during that time. So from the 15th - 15th if you had all your Waves in your lite client, you'll receive a full distribution weighted to your amount of Waves.

If for example you moved all your Waves from Bittrex to the client on the 14th, and then wanted to get the airdrop on the 15th, well, for those other 29 or 30 days your balance was zero. So you won't get a distro.