Just in time for a Celtics play-off basketball game Monday night, AT&T said it is throwing the switch on a network upgrade that will expand mobile Internet coverage at TD Garden, the arena where the Celtics play their home games.

As any fan worth his or her smartphone can tell you, half the fun of attending a game or a concert these days is posting social media updates while an event is in progress. Or maybe a fan just wants to send a tweet or a text.

But here’s the rub. Suppose Celtics star Kevin Garnett makes a spectacular play and, suddenly, many of the roughly 18,600 fans in attendance all want to tweet at once. Such an outburst of concentrated activity can overwhelm a building’s wireless coverage. And one result is that tweets can lag, and texts are no longer real-time.

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To reduce the likelihood of such a calamity, AT&T said it has spent the last four months installing a new 4G LTE-capable Distributed Antenna System, or DAS, to better serve Garden attendees who use AT&T as their wireless carrier.

‘‘We know that reliable wireless coverage at the TD Garden is very important to Boston sports and music fans, who want to text and post social media updates while attending events,’’ Steve Krom, vice president and general manager of AT&T New England, said in a statement. ‘‘We want our customers to have a great network experience, and providing enhanced wireless coverage at the Garden is just one way that we’re investing in Boston.’’