I am starting a new position next month and I have been kindly invited to a team lunch. I would love to go, but due to the month of Ramadan I will be fasting during the day - no food or drinks allowed. I am not sure if it would be rude to decline their offer. I could ask them to delay it by 1 month but then that is no longer a welcome lunch.
Observing your religion shouldn't be viewed as being rude.
Simply respond and explain the situation to them while thanking them for their offer.
You can certainly suggest delaying for a month, and they should be happy enough to do this (other plans permitting).
They may also offer to go for an evening event instead of a daytime one. They might even go further in going ahead with the lunch event, but choosing not to eat or drink out of respect for you (this is something I'd happily partake in).
As Snow suggests, observing your religion in this way isn't rude - and asking for a delay isn't a big deal but if it's not feasible that doesn't mean you have to cancel or postpone the lunch.
The point of such a "welcome lunch" isn't really about you eating or drinking things - it's about getting to know your new colleagues in an environment that is less formal than the office or a meeting. You could quite easily fulfill the purpose of doing this by attending and not eating or drinking.
The purpose of the welcome lunch is not really the food, it is to welcome you to the company.
Consider accepting the invitation but explaining that you will be in the middle of a fast and will not be eating.
Perhaps in light of that, the welcome lunch will be changed into some other kind of welcome event that doesn't involve food.
I suggest that it's not necessary to explain why you can't make it to lunch. You can decline and suggest an alternate date.
For example, you might say "I'm grateful for the warm welcome you're all giving me here at CompanyName. Your offer for the welcome lunch is very kind, but I'm afraid I won't be able to accept until after [specific date]."
There are some things that don't need to be discussed in the workplace, such as medical issues, home life, religion and so on.
For example, you might not be able to attend a lunch because:
- You're fasting for religious reasons
- You're preparing for a medical procedure that requires a specific diet
- You want to remain available so that you're on call to tend to an ailing relative
- You have a special diet because you're trying to make some weight loss goal
- etc etc etc
The key is that you don't need to explain why you're declining the invitation. Explain that you won't be able to go, thank them for the offer, and also give a timeframe when you would be able to in the future. If they can't reschedule, that's fine.
None of this is to say that one should hide their religion. It's simply that it's not necessary to explain your reasons when declining an invitation if you don't want to, and even moreso when the reasons are related to things that are typically left private at work.