Bienvenue is among the many believers in this year’s Celtics team, and that belief has translated into a financial boost for Boston businesses and for the city’s hotel occupancy rate. “A year ago even, we were probably running at about 45% to 50% occupancy and I’d say, this month, we’re going into the high 80s — if I had to guess,” said Martha Sheridan, president and CEO of the Greater Boston Convention & Visitors Bureau. With at least two NBA Finals games guaranteed to be played in Boston, the month of June is expected to be a record-breaker for the city’s hotels, bars and restaurants.