What are we commanded to guard in Exodus 12:17 and how do we obey that commandment?

Answer From The Annual Reading Schedule - Week 16 Notes For Exodus: The unleavened bread. Some English translations say to guard or keep or observe the feast of Unleavened Bread. However, the Hebrew literally says, וּשְׁמַרְתֶּם אֶת־הַמַּצּוֹת (u'shemartem et-ham'matstsowt) which translates literally to and'you.shall.safeguard at-the'unleavened.bread – or "You will guard the unleavened bread." The word "feast" is not in that phrase. So what is being commanded to be guarded is the unleavened bread. Guard it from what? Well, from leavening. How do you do that?

Well, remember in Exodus 12:34 they had their dough and kneading bowls with them and they were rushing to leave Mitsrayim (Egypt). If they weren't permitted to cook on the 1st and 7th days, how would they be able to guard the unleavened bread? Certainly their dough could have become leavened if they could not cook on the 1st and 7th days, and they may not have had hardly any unleavened bread to eat. They had to be permitted to cook on those days because otherwise they would not have been able to be obedient to the command to guard the unleavened bread and eat unleavened bread for seven days.

So the previous verse, Exodus 12:16, is actually telling you how to guard the unleavened bread – by ensuring you cook and not let things sit around. That is why cooking is permitted during the feast of unleavened bread on the 1st and 7th days when we are told to rest – because that cooking is part of being obedient to the command to guard the unleavened bread, and to eat it seven days.

It is for this reason that I do not see Exodus 12:16's permission to cook as applicable to any other feast days throughout the year – only for the Feast of Unleavened Bread. That permission to cook is specifically related to guarding the unleavened bread and ensuring that there is not any leavened bread permitted to form by left over dough, etc., and to ensure that everyone does have enough unleavened bread to eat since leavened bread is not permitted.

Also, we can see in Exodus 12:39, that they were cooking and had no time to prepare food in advance before leaving.

Final Notes/Comments

I do not believe the permission to cook in Exodus 12:16 means even if the 1st or 7th days of the feast fall on a weekly Sabbath. In the case of the weekly Sabbath, I believe the requirement to keep the weekly Sabbath Qodesh (Set-Apart/Holy) supersedes this permission to cook and that you can never cook anything on the weekly Sabbath.
I do not believe Exodus 12:16 means you can cook "as much as you want" – I believe based on how Exodus 12:16 is worded it's only what's "necessary" for your immediate needs to eat that day only – and I consider it a possibility that Yahweh might actually be referring to a permission to cook the unleavened bread only (to safeguard the unleavened bread) and that maybe cooking other things besides the unleavened bread should be done when it's not the days of rest. However, since I can't prove this to myself, I sometimes do cook other things besides the unleavened bread on the days of rest as long as it's not a weekly Sabbath.
The only exception where you might cook possibly a little more than what you need that immediate day based on Exodus 12:16 might be if the 7th day of the feast was on the 6th day of the week and you had to prepare for the weekly Shabbat – but in that case you should be probably cooking on the 5th day of the week as much as possible to minimize what would be needed to be cooked on the 6th day of the week (7th day of the feast) so that you are ready for the weekly Shabbat the day after the feast ends without making the 7th day of the feast on the 6th day of the week a "cook day" since it's a day of rest and only intended to cook minimally, only what's needed, and nothing more. So if the weekly Sabbath falls one day after the 7th day of the feast, and you have back-to-back Sabbaths, maybe cook other non-bread items on the 5th day of the week so that two days later you have food on the weekly Shabbat without needing to cook on the 7th day of the feast that fell on the 6th day of the week.