In English, we have a few ways of referring to time, relative to the current time. Mandarin is similar, and has a few patterns.
天 (Tiān) is day (although it can relate to the heavens, God, and much more)
Although 每个天 (Měi gè tiān) is literally "every (or each) day", doubling-up the character as 天天 (Tiāntiān) can also be used for every day
今天 (Jīntiān) is today
昨天 (Zuótiān) is yesterday
明天 (Míngtiān) is tomorrow (and roughly translates to "bright day")
前天 (Qiántiān) is the day before yesterday (and roughly translates to "in front day)
后天 (Hòutiān) is day after tomorrow (and roughly translates to "rear (or perhaps behind) day"