The Seville tourist office is a bit vague. From its website:
On national and local holidays, public transport systems run and you will find some museums, monuments, information offices, shopping centres, etc. open. However, these will often have special opening times and/or conditions, given that there may be activities taking place in the area at the same time (fairs, open-air celebrations, religious events, etc.), so you should find out in advance to plan your trip correctly.
New Year: 1 January
Epiphany: 6 January
Andalusia Day: 28 February
Maundy Thursday and Good Friday: date varies year to year (March-April)
Workers’ Day: 1 May
Assumption of the Virgin Mary: 15 August
Columbus Day: 12 October
All Saints’ Day: 1 November
Spanish Constitution Day: 6 December
The Immaculate Conception: 8 December
Christmas: 25 December