Solemnity of the Immaculate Conception of the Virgin Mary. It is a holy day of obligation in the United States and several other countries meaning all Catholics are bound to attend Holy Mass. Failure to attend Mass, without a sufficient reason, is indeed a Mortal sin.
CCC 1861: "Mortal sin ... If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back."