When you are guest it is important to be a gracious guest. At the same time, having resolve to uphold a commitment is a good thing. Can you have steak on a Wednesday if it is offered to you? The answer may be yes, within reason. When it's not insulting to pass on something, do it. Such as, only have water not soda, beer, or sweet drinks. Pass on desserts when possible, especially in large group settings where it is easier to do so.
In addition, pass when your brother, who knows you are doing Exodus 90, offers you something. He understands, thus it's ok to pass. But yes, if you are at a dinner with your grandma and she made steak on a Friday just the way you like it, please eat the steak. In turn, have greater resolve to commit to the other disciplines.
Watch out for Pride.
Never should your passing on something purposely be a means of drawing attention to yourself. One is not growing in holiness if they are subtly boasting about how holy they are for abstaining from beer and sweets. Satan wants to rob you of graces whenever possible. Making you act pridefully when fasting or abstaining is one of the ways he does this. Complaining is another. Be humble and as silent as possible when you are abstaining. Further, be grateful for the grace given to you to abstain from good things for the salvation of others.