Blair County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 122,822. Its county seat is Hollidaysburg, and its largest city is Altoona. The county was created on February 26, 1846, from parts of Huntingdon and Bedford counties. The county is part of the Southwest Pennsylvania region of the state.

Hollidaysburg is a borough in and the county seat of Blair County in the U.S. Commonwealth of Pennsylvania. It is located on the Juniata River, 5 miles (8 km) south of Altoona and is part of the Altoona, Pennsylvania, metropolitan statistical area. In 1900, 2,998 people lived in the borough, in 1910, 3,734 lived there, and in 1940, 5,910 residents were counted. The population was 5,641 at the 2020 census. Coal, iron ore, ganister, and limestone are found in the vicinity. In the past, the borough had foundries and machine shops, a silk mill, car works and classification yards.