*All quotes are taken from The Norton Shakespeare, based on the Oxford Edition.

In Act I, Scene 1, it is revealed that Olivia has vowed celibacy for seven years while mourning the death of her brother: "The element itself till seven years' heat/Shall not behold her face at ample view,/But like a cloistress she wil veiled walk/And water once a day her chamber round/With eye-offending brine--all this to season/A brother's dead love, which she would keep fresh/And lasting in her sad remembrance." (I.i.25-31) Olivia is most likely using this extreme excuse to deter Orsino from pursuing her.
When Olivia meets Cesario (Viola), she decides that she can stop mourning for the loss of her brother because she has found a new love interest.