I was hoping to get your opinion on celebrating holidays (such as Thanksgiving, Mothers Day, Birthdays) in the following situations:
1) Muslim only gathering
2) Mixed gathering (Muslim and Christian)
Thanks for your help!

All praise be to Allah, and may his peace and blessings be on the last and best prophet and messenger, Muhammad.
The feasts that may never be celebrated by consensus are the religious feasts of non-Muslims. Also, we may not celebrate innovated religious feasts, even if they were innovated by Muslims. The national holidays are to be avoided since they are recurrent and public like al-Fitr and al-Adha, and thus would fall under the Prophet’s statement:
“ إنَّ الله أبْدَلَكُم بخَيرٍ مِنهُما: عيد الأضحى وعيد الفطر ” ‘Allah has given you in their stead something better: Eid al-Adha and Eid al-Fitr.’ (Narrated by al-Nasa’i)
The personal celebrations are controversial. These would be like personal birthdays and wedding anniversaries. They are in a way competing with the two Islamic feasts of fitr and adha and in a way different, since they are not public celebrations. Based on that, even if I prefer for you to avoid such feasts, I find it hard to say it is haram to celebrate them.
If you decided to celebrate these, I would advise the following:
• Segregation between the genders; Women should all be observing proper hijab (of dress and conduct) if complete segregation is not affordable. In such case men would be on one side of the room and women on the opposite side.
• There should be no music played; Songs accompanied by percussion instruments are allowable.
• To have non-Muslims of the same gender for a good cause is permissible.
Allah knows best.