Holy Days of Obligation are days of such importance that they are intended to have the same liturgical strength as Sunday. Therefore it is expected that all the baptised will participate in their celebration.
Sunday is the primary Holy Day of Obligation. In Australia there are only two holy days that may fall on a weekday: the Nativity of Our Lord on 25 December and the Assumption of the Blessed Virgin Mary on 15 August. The rest have been moved to a Sunday: the Epiphany (6 January), the Ascension, and the Body of Blood of Christ. Additional holy days of obligation may be celebrated in other parts of the world including Mary the Mother of God (1 January), the Immaculate Conception (8 December), St Joseph (19 March), Saints Peter and Paul (29 June) and All Saints (1 November).
Image Attribution - The Assumption of Mary, Reubens, Antwerp Cathedral