Select the most appropriate idiom for the given situation focusing on the underlined words.
I invited them to my anniversary dinner but they had other plans, so they decided to politely decline with the implication that they may come at a later date.

The correct option is To take a rain check.

Explanation:
An idiom is a phrase or expression whose meaning cannot be understood from the literal definition of its individual words. Let's analyze the meaning of the given options to find the one that fits the underlined situation ("politely decline with the implication that they may come at a later date"):

1. To take a rain check: This idiom is used to politely decline an offer or invitation, with the suggestion that it can be accepted at a later, more convenient date. This directly matches the situation described in the prompt.
2. To spoil a moment: This means to ruin a nice or special occasion or experience.
3. Call it a day: This means to stop working or doing something for the rest of the day.
4. Took it with a grain of salt: This means to view a statement or piece of information with skepticism or not to take it literally or too seriously.

Therefore, the idiom that perfectly aligns with the given scenario is "To take a rain check".