Modern pagans (Neopagans) like to believe that the ancients celebrated their festivals on set dates on these calendars. This is patently impossible. The major reason is that these calendars did not exist for them!
The Druids existed in Northern Europe long before Julius Caesar. Records seem to confirm that they retreated from the Romans to concentrate in Britain, Scotland, Wales and Ireland. If not a retreat, it was a migration. Wicca was invented in the late 1800's by Aleister Crowley and Gerald Gardner. Both of them were members of the Ancient Order of Druids before developing their systems. Anyone following their practices are NOT following the 'ancient' ways. (This is nothing more than stating facts and not any opinion about the validity of those practices.)
The Druids were keen observers of the movements of the Sun (masculine energies) and Moon (feminine energies). They observed the cycles of the day and that of the year. The date of the Equinoxes and the Solstices were easily set. That is the basis of four of their holy days (holidays). They also could determine what is called the "Cross Quarters". The halfway points between the first mentioned dates. Using the current Gregorian Calendar this puts the dates for the Pagan Holidays as follows. Imbolc or Candelmas (a Roman Catholic holiday as well) occurs on February 2 or 3 depending on the Sun. The festival called Beltaine (Beltane) falls on the 3rd or 4th of May. Lughnasadh usually happens on August 2 or 3 by Solar reckoning. Samhain falls on November 4th or 5th.
"Modern" Calendars
The Roman calendar had 10 months before Julius Caesar. January was named for the God Janus who had two faces. One facing forward to the future and the other looking towards the past. February was named for the Februa, purification festival, that occured on February 15th. March was named for the God of War, Mars. April comes from the Latin word for opening. This is because April used to be the second month of the year before January and February were added to the Roman Calendar in about 700 BCE. The buds were opening (as they do sometimes in February now). May is named for the Goddess of Fertility, Maia. June was named for Juno, the wife of Jupiter. The rest of the months were simply numbered. (Latin prefixes for 7,8,9,10 - Sept-, Oct-, Nov-, and Dec-). July was added by Julius Caesar to honor himself. Augustus decided that if Julius could do it, so could he.
Gregorian Calendar
The problem with the Julian Calendar was that it did not consider the actual time it takes to revolve around the Sun. The year gradually "moved" in relation to the seasons. Seasonal festivals held according to the Julian Calendar stopped being "seasonal". Pope Gregory XIII decided that something had to be done. He devised the "modern" Gregorian Calendar. Thursday, October 4, 1582 was followed by Friday October 15, 1582. Eleven days simply disappeared. The Pope imposed this calendar on his subjects because the Pope still had an army. The "Eastern" Church resisted this change at first. The rest of the world simply did not care. A similar effect is seen today in the Muslim calendar that has a year of exactly 360 days.
Conclusion
The Ancient Order of Druids was founded as a "fraternal" organization in 1781 (the group that the founders of Wicca were in). They cannot be the bearers of "ancient" knowledge. Witchcraft was illegal in Britain until 1952. (Except for the time when Henry VIII decreed that no one could be a witch...) Taking all this into consideration, we can see that the ancients could NOT have celebrated their festivals on the dates of the current calendar. They did not even exist until Julius Caesar set the number of months. They were not fixed to the seasons until Pope Gregory did that in 1582.



