user1142: A stake pool always has an "active stake" and a "live stake". "Active stake" refers to the amount of ADA that is used for rewards calculation in the current epoch. "Live stake" is all ADA that is delegated to the pool, also counting ADA that was delegated/undelegated after the "active stake" snapshot for the current epoch.
At which point is the snapshot for the "active stake" taken? Is it right when the new epoch starts? And will this happen at the exact same moment for all stake pools?

user281: Staking snapshots are taken at the beginning of each epoch and a stakepool's active stake is whatever was delegated to it at the beginning of the prior epoch or during the epoch before that.
Any new or changed stakepool delegations take effect at the beginning of the next (1st) epoch after they are made (they become part of a stakepool's live stake).
In the (2nd) epoch after that the delegation is used to determine the stakepool's probability of rewards and is part of the stakepool's active stake.
In the next (3rd) epoch since delegation rewards are calculated based on the number of times the pool became a slot leader.
In the last (4th) epoch rewards are distributed to the delegator.
In summary


Delegation changed
Snapshot taken
Slot leaders chosen based on delegation
Rewards calculated
Rewards distributed


"Cardano Staking: Practical Information" provides a more detailed explanation

1142: <p>A stake pool always has an &quot;active stake&quot; and a &quot;live stake&quot;. &quot;Active stake&quot; refers to the amount of ADA that is used for rewards calculation in the current epoch. &quot;Live stake&quot; is all ADA that is delegated to the pool, also counting ADA that was delegated/undelegated after the &quot;active stake&quot; snapshot for the current epoch.</p>
<p>At which point is the snapshot for the &quot;active stake&quot; taken? Is it right when the new epoch starts? And will this happen at the exact same moment for all stake pools?</p>
 281: <p>Staking snapshots are taken at the beginning of each epoch and a stakepool's active stake is whatever was delegated to it at the beginning of the prior epoch or during the epoch before that.</p>
<p>Any new or changed stakepool delegations take effect at the beginning of the next (1st) epoch after they are made (they become part of a stakepool's live stake).</p>
<p>In the (2nd) epoch after that the delegation is used to determine the stakepool's probability of rewards and is part of the stakepool's active stake.</p>
<p>In the next (3rd) epoch since delegation rewards are calculated based on the number of times the pool became a slot leader.</p>
<p>In the last (4th) epoch rewards are distributed to the delegator.</p>
<p>In summary</p>
<blockquote>
<ol start="100">
<li>Delegation changed</li>
<li>Snapshot taken</li>
<li>Slot leaders chosen based on delegation</li>
<li>Rewards calculated</li>
<li>Rewards distributed</li>
</ol>
</blockquote>
<p><a href="https://cardanians-io.medium.com/cardano-staking-practical-information-3c86cbc73bd4" rel="nofollow noreferrer">&quot;Cardano Staking: Practical Information&quot;</a> provides a more detailed explanation</p>
