Pagan holidays and their Christian connections:

Samhain / Hallowe'en (All Hallow's Eve) a primarily Celtic holiday: All Saint's Day.

Winter Solstice / Yule / Saturnalia: Christmas

Imbolc / Brigid's Day (Celtic): Candlemas (and Groundhog's Day)

Spring Equinox / Ostara: Easter

Bealtaine (Celtic, once again) / Walpurgusnacht (Germanic): May Day

Midsummer / Summer Solstice / Litha: St. John the Baptist Day

Lammas / Lughnasadh (Celtic): Feast of St. Peter's Chains

Autumnal Equinox / Mabon: General harvest festivals