It depends on when the holiday is/was declared.
If the holiday status was set prior to the time she spoke, you can use is (although will is also often used in the same context). The holiday-status of the day isn't in question; she just doesn't know what that status is.
- If a federal holiday falls on a Saturday, the preceding Friday is a holiday.
- officeholidays.com (emphasis, mine)
If the holiday status is uncertain at the time she spoke, use will.
Often the uncertainty does not matter and you will be able to proceed as if tomorrow will be just like today
- Dennis V Lindley (emphasis, mine)
If you are working in Washington on this day (even if you normally are assigned elsewhere), it will be a holiday. If you are on travel elsewhere, it will not.
- US Dept of Commerce (emphasis, mine; the uncertainty stems from not knowing the location)
Will can also denote certainty, such as when one is declaring a holiday, or declaring that a particular day is a holiday. The Washington example above can be argued to fit this interpretation if the quote is taken as an authoritative statement. In any case, this doesn't apply to your example since she "doesn't know".
- Tomorrow, Washington's Birthday, will be a holiday in all departments of the University.
- The Harvard Crimson (emphasis, mine)
Use can be if there is an element of choice about the day's holiday status.
- Any Day Can Be a Holiday
- Huffpost (emphasis, mine)
So all three variants are possible. Since the original has an element of uncertainty, the will version is arguably closer to the intent than the is version, though the form of uncertainty is somewhat different.