Is it based on daily average of just a snapshot of Monday 11:59 UTC
Inflation occurs when somebody (anybody) transacts the Inflation Operation and it has been at least 1 week since the last successful Inflation Operation. The calculations are performed at that time.
The inflation pool is
(number of lumens in existence)*(weekly inflation rate) + fee pool
This amount is shared pro-rata amongst all accounts which meet the minimum voting threshold. That is, the cumulative amount of lumens held by accounts voting for an account must exceed
(number of lumens in existence)*.0005.
Full details of this process are documented in the Stellar dev guide.