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American B1/B2 visas are valid for travel from the very day it is issued.
A visa issued for a single entry (denoted on the visa under “Entries” with the number 1) is valid, or can be used from the date it is issued until the date it expires to travel to a U.S. port-of-entry one time.
A visa issued for multiple entries (denoted under “entries” with a certain number (2, 3, etc.) or “M” for multiple/unlimited entries) is valid, or can be used from the date it is issued until the date it expires to travel to the U.S. port-of-entry as many times as your visa states, provided that:
You do not need to request it explicitly. The consular officers typically understand that people travel some days before their event and that people may have other minor reasons for travel in addition to the main reason for applying.
Make sure you carry all the documents related to your travel on you, basically the documents supporting your trip.