。
I'm going to visit Mary the day after tomorrow.
Literal
I [topic-は] day-after-tomorrow Mary [object-を] visit [intention-つもりだ].
~つもりだ ('intend to, plan to') is a standard intention expression. 明後日 ('the day after tomorrow') is a useful time word alongside 明日 ('tomorrow') and 一昨日 ('the day before yesterday'). 訪問する ('to visit, call on') is a formal Sino-Japanese verb; the more casual alternative is 訪ねる.