Inauspicious Days (Baden) for the Earth-Dog Year 2145 (2018 ) according to the Du-Khor Be-kar - Astrology of the Kalachakra
According to the Tibetan Calendar there are certain days when a "baden entity" is present. Those days are inauspicious for raising new prayer flags. A baden senpo is a demon of the naga class of elemental spirits. Raising flags when that energy is active can actually have a negative effect.
The dates below are days that are not good to raise prayer flags.
March (2018): 21
April: 1, 13, 19
May: 13, 16, 18, 29
July: 5, 19, 31
August: 15, 27
September: 10, 22
October: 4, 19, 31
November: 14, 26
December: 11, 23
January (2019): 3, 6, 19, 30
Please note that the dates are just days that are not good for initially planting the flags. Some people have written to ask if they need to take down prayer flags on these days. Certainly not; once they are up it's okay to keep them up until they disintegrate or until they are replaced with new flags.
As far as what days are best for raising new prayer flags: that really depends on a person's individual astrology. Without consulting an astrologer I'd say the best time to put up new flags are in the mornings on sunny, windy days. Mondays are considered especially good. Offering prayers can be done before raising the flags. Two excellent offering prayers can be found on this website on the page "Buddhist Smoke Offering Ceremonies."