Pesach was originally the celebration of the commencement of spring. It took the form of a week-long abstinence from leavened products, with a special gathering on the first and seventh days. It then became the occasion at which the events of the Exodus of the Israelites from slavery in Egypt were retold.
Pesach falls on the full moon of the month of Nisan, which is in March or April. It is exactly six months before, and either six or seven months after Sukkot (in order to keep the agricultural festivals connected to the time of year they celebrate, an extra lunar month is inserted into the calendar seven times every nineteen years immediately prior to the month of Nisan).
The prohibition against eating leavened products means that Pesach is a time for spring cleaning. There are many elaborate rituals relating to the removal of such products from one’s home prior to the festival. On the first night of Pesach a ritual meal known as a Seder (which means ‘order’) is held, usually in the home. Traditionally this is a family occasion at which the story of the Exodus is related, using a book called a Haggadah (which literally means ‘telling’) and a number of symbols and rituals all designed to assist the person leading the Seder in their retelling of this ancient legend. In recent years, the idea of a communal Seder, organised under the auspices of a congregation, has gained in popularity. Services are held in synagogue on the first and final day of Pesach.
The level of adherence to the Passover dietary regulations varies among Liberal Jews, but the observances are mainly the same.
The prohibition against leavened products means that there are a number of foods unique to Pesach. Most prominent among these is matzah – the unleavened bread that takes the place of ordinary bread during the festival. There are also many different varieties of cakes and biscuits that are made using ingredients other than wheat flour – almond macaroons, cinnamon balls etc.
Rather than retyping the explanation of the origins of the halakhah concerning Matzot Ashirah I would point you to this excellent blog post. The conclusion we can draw from the Shulchan Arukh is that while for Sephardim Matzot Ashira are permitted during Pesach, “the mitzvah of matzah cannot be fulfilled with Matzot Ashirah.” As is the case for several of the laws pertaining to Pesach, the Ashkenazi interpretation is even more stringent and states that only those who are elderly and infirm should eat them at all.
 
For us Liberal Jews, the most interesting aspect of the discussion is the reason, which is given for the ruling that “the mitzvah of matzah cannot be fulfilled with Matzot Ashirah.” It is that we are commanded to eat “the bread of poverty” (as stated in Deuteronomy 16:3). So the question is whether in the 21st century, Matzah Ashirah can be considered to be “the bread of poverty.” How would we answer that question?
 
One indicator may be the cost of the matzah – of course, this varies greatly depending on where you are in the country but generally speaking Matzah Ashirah is significantly more expensive than regular matzah. However, by this logic, we most certainly shouldn’t be using Shmurah Matzah, which is even more expensive but which many authorities claim is the only matzah that one may use to fulfill the obligation of eating matzah at the seder. And if we were really trying to eat “the bread of poverty,” it would probably have to be a supermarket-brand value loaf of white sliced bread – I think we would all agree that this would feel rather odd at a seder. So if cost cannot act as useful guidance, it seems to be left to our intention when eating the matzah and our personal understanding of what it means to eat “the bread of poverty” which must guide us in knowing if one has fulfilled the mitzvah.
 
So have you fulfilled the mitzvah of eating matzah at the seder even though it was with Matzah Ashirah? From a Liberal Jewish perspective, I would say it depends – if you reflected on the meaning of eating matzah during the seder, the answer surely must be yes. But if you just ate the matzah to tick a box on some imaginary seder checklist, the answer is probably no and it would have likewise been no had you eaten Shmurah Matzah or any other regular Matzah.