Learn Finnish: Jos sää on kaunis, viemme tarjoilun vieraille ulos. - If the weather is beautiful, we will take the catering outside for the guests.

Finnish doesn't have a separate future tense. You simply use the present tense (like viemme) and let the context show that it will happen later. Here, the jos (if) clause naturally sets up a condition for a future event.

Why is 'for the guests' translated as vieraille?

This is the plural allative case, which often translates to 'to' or 'for' someone. The basic word for guest is vieras. To build this form, we take its stem vieraa-, add the plural marker -i-, and attach the ending -lle, giving us vieraille.

Why does tarjoilu (catering/serving) have an -n at the end?

It acts as the object of the verb viemme (the thing being taken). Because we are talking about taking the entire catering outside as a complete action, it takes the -n ending rather than the partitive case.

Could I use ulkona instead of ulos for 'outside' in this sentence?

No, because Finnish strictly distinguishes between being in a place and moving to a place. Ulkona means you are already statically outside. Since the verb viemme (we take) involves movement from inside to outside, you must use the directional word ulos.