In the Cardano ecosystem, time is divided into epochs of 5 days. Epochs are used as a specific accounting period for which rewards are paid. Between epochs, a snapshot occurs in which the ADA coin balance of the accounts is scanned. The protocol knows what coins are staked in a given epoch and can calculate the reward based on this. The Staker is not entitled to a reward on the first day after delegating coins to the pool. 15-20 days may elapse between the time ADA coins are delegated and the first reward received. Let's give epochs an artificial name from the point of view of the stacker who wants to delegate ADA coins to the pool. The staker delegates coins in an epoch called DELEGATION. This may be a few hours before the snapshot, or it may be a few days. At the end of the epoch, a snapshot is taken and the coins must wait one epoch before the pool uses them. Let us call this epoch STAKE-REGISTRATION. In the next epoch, the pool can use the coins. We can call this epoch STAKE-USED. The staker is entitled to a reward for this epoch. Suppose the pool has created at least 1 block and is eligible to receive a reward. The next epoch is used to calculate rewards, let us call it REWARD-CALCULATION. Note that the rewards are only calculated but not yet distributed. Finally, in the next epoch, the staker receives the first reward. Let us call this epoch REWARD-DISTRIBUTION. Let's put the epochs in order: DELEGATION, STAKE-REGISTRATION, STAKE-USED, REWARD-CALCULATION, and REWARD-DISTRIBUTION. Staker has to wait at least 3 middle epochs for the reward. The reward may come at the end of the last epoch. Once you receive the first reward, you will receive the following one each epoch. Let's add that thanks to snapshots, earned rewards and new ADA coins in the account are automatically counted and used in staking. If you delegate ADA coins to another pool, you will still receive rewards from the old pool.
