This is a bit tricky because "Tag" can refer to both "day" and "tag" (as in a name tag), and the word for "day" can be feminine in some contexts. But when referring to a day of the week or simply "day" in general, "Tag" is always masculine.
Here are some examples:
* Der Tag ist schön. (The day is beautiful.)
* Am Montag ist der Tag frei. (On Monday, the day is free.)
* Der Tag der offenen Tür ist am Samstag. (The open house day is on Saturday.)