United Airlines expects its busiest spring break ever this year, even as travelers say they are concerned about rising costs. The carrier said Monday it expects to serve more than 21 million passengers from March 8 to April 21 — up nearly 10% compared with last year. That equates to more than 200,000 people per day on average during the period — about 15,000 more people flying per day than last year. United said it expects the busiest travel day to be Friday, March 22, when more than 500,000 passengers are scheduled to travel via the carrier. Likewise, the week of March 25 will be the busiest travel week, with about 480,000 people flying each day that week, on average. The travel group Expedia separately reported last month that the April 8 solar eclipse appears to be driving some of the biggest surges in bookings, with locations in Texas — the state with one of the…