Is it permissible to send messages like "I wish you well" or "I wish you the best" to Christians whom I know on their holidays, with the intention of wishing them guidance, knowing that congratulating them on their holidays is forbidden?

Congratulating disbelievers on their holidays is forbidden, and sending them a message wishing them well on their holidays is considered a forbidden congratulation. However, if this message is sent outside of their holidays with the intention of inviting them [to Islam] and guiding them, then there is no harm. The Prophet, peace and blessings be upon him, used to pray for the Jews for guidance and peace of mind, as narrated in the hadith (definition) of Abu Musa. Therefore, supplicating for a disbeliever for guidance or peace of mind is permissible.