Edoardo translates "to blow away" with "to rip off", which in Italian sounds the most suitable translation (> to rip a garment off a person); I don't agree instead using "spazzare" (to sweep off), as it is used for large areas (such as lands and the sky -- eg. to sweep the clouds off the sky).
However, when we translate "it was so windy" we tend to transform the adjective in the noun "c'era così tanto vento", starting a Consecutive clause ("so that..."), whose subject is "IT" . Now the main subject is "That day" : so who is the real subject? the day rips off the backpack? There must be a mistake...
Therefore, Federica makes the best syntactical choice, using the passive form, letting us realize that obviously is the wind which makes the job (not "the day").
However (!) I still don't like very much the choice of the Passato Prossimo, which is used when we want to connect a past event as an explanation of the present situation (so that we call it "prossimo" = near to us). I wonder how that story is important to us, so I just think it is the typical storytelling to amuse someone, and in this case the events narrated go in the Past Perfect > Passato Remoto (which is your original tense).
CONCLUSION: we can now create the best "hybrid":
- Quel giorno c'era così tanto vento che il mio zaino fu strappato via.
- or [Quel giorno era così ventoso che ...]