When the IOTA foundation performs a network-wide snapshot, they define a milestone index (i.e. the index of a transaction issued by the Coordinator). Nodes who have not seen that milestone (because they are not synced) will not be able to verify the snapshot. All other nodes that have this milestone can verify it, since it confirms the same transactions for everybody.
Users are notified about snapshot via discord or GitHub about snapshot time.
Is there an automatic way to notify users via wallets that snapshot is going to occur on this date.