Our 2 indoor cats (same litter) live with three people, one of the three being me. They're fully accustomed to our presence and are very affectionate, asking to be petted, sleeping with us in beds, comfortable with us walking around them, etc.
We sometimes have council workers come over for an hour or two, to do some odd jobs such as plumbing and whatnot. We also have relatives come over once or twice a year from overseas, and their prolonged stays at our house have a big impact on our cats.
Every time a new person comes to the house, the cats respond by immediately hiding under beds or behind or inside wardrobes, which is irregular to their regular napping spots such as platforms by windowsills, on bed, etc. It is clear to me they are not aware of what is going on, and they're too afraid to investigate the new voices, so they want to be hidden. They stay there for several hours even after the workers have left.
In case of relatives who stay over for weeks, the cats sometimes stay in hiding for 1-2 days straight. They do get out at night to eat and drink, but they go into hiding again the next day. They get used to the relatives after like a week, but in my opinion the warming up process takes too long, it's not normal.
From what I've observed this is a bit like dogs and fireworks, in our current state the 'big scary strangers' visiting the house every so often is inevitable, but we are worried about how the cats respond to this entire ordeal every time.
Is there anything we can do to discourage our cats' fear of strangers?